How do you call the person who is ignorant/unskilled in his work?

I dont think we have a specific word. The word incompetent or useless at job gets thrown around

Dilettante?

a moron. kidding aside. inept (opposite of adept). incompetent. inutile (that's a strong insult though).

Maybe saying that someone is “all thumbs”? This means they make mistakes frequently, but it isn’t really an insult per se, if anything it’s kind of pitiful. I’m not totally clear from your description whether the crucial part is that this person can’t do anything right, or if the crucial part is that they deny it. Because there are all kinds of words for someone who is just an idiot who doesn’t realize they’re an idiot. Calling someone like this “a horse’s ass” is a personal favorite. The word “jerk” used to tend to mean something like “a clueless idiot” more than “a rude person,” which is generally what it most often means now.

I like the other responses. I just wanted to say that your title should say "*What* do you call *a* person..." In English, we always use "What do you call" to ask for the names of things.

"*What* do you call," not "*how* do you call."

A 'bodger'. Bodging is actually an obscure term from woodworking but has been re-purposed. So if someone is a bodger they do a bad job, a 'bodge job' is the result of their work and you can also say 'this work has been bodged'. A bodged repair, for example, might have a lot of ugly glue and duct tape holding it together instead of being done properly. When something breaks you might hear a skilled worker say 'I can bodge this until I can get the proper parts to repair it'. I suspect this is very much British English!

there is a phrase that might at least fit the tone of your original statement and it’s someone having “their thumb in/up their ass.” this doesn’t necessarily mean ignorant or unskilled, it mostly means lazy or nonattentive, not doing the work they’re supposed to. example: “Where’s my order? Whoever’s making my food must be back there with their thumb up their ass.” for specifically unskilled, someone else brought up “all thumbs” but i’ll also add in “dead weight,” when someone isn’t contributing to the work or is doing a bad job so it feels like they’re a burden and just wasting time/space “I had to do the work of two people since my coworker was just dead weight.”

Craft words together until you find this comparison a word like this is not a thing in English. The way you get worse than like “Motherfucker” is like “Your visage is so hideous that the last time you had a portrait taken it required neither hammer nor nail as it hung itself” which is wordy and slightly old fashioned but offensive nonetheless and “What you need is a mirror not a painting. It will showcase the horror I see before me far better than I ever could”(alphinaud levellieur ffxiv shadowbringers). TL:DR; you only get as offensive as you claim to be trying to be by comparison. Not by single word insults.

Useless

Incompetent. (You said in a comment you’re looking for something impolite- this would be a harsh thing to call someone.)

Cack-handed is very familiar to me but it could be regional (British English here). But I would say it has a more specific meaning of clumsy than generally incompetent and useless.

I use either “incompetent” or “inept”.

I'd suggest buffoon as something that's quite close. It implies stupidity, lack of skill, and messing things up. For example, "He was such a bumbling buffoon. He'd failed upwards throughout his career despite his numerous flaws."

A number of people have mentioned cack-handed. In British English this can be used for anyone who isn't very good with their hands, who does things in an awkward or untidy way. For example "I tried to put up a shelf for my books, but I'm so cack-handed that it isn't straight." If you want to specifically describe someone who does manual work for money - like an electrician, a plumber or a builder - the kind of person you would pay to do some work on your house, then in British English you can use the word cowboy. It gives the sense of someone who does poor quality work but who is also dishonest. For example - "I had my roof fixed, but it still leaks. The builder was a complete cowboy and now he won't return my calls "

OK! You're going to have to give us an American English phonetic spelling of that Russian word you mentioned! Please. I can imagine it would come in very handy! If I'm referring to someone who thinks he is good at his or her job, but, in fact, isn't, I say that that person has "delusions of competence".

I'm surprised nobody else has suggested "dipshit." I feel like that's on the level of offensiveness you're going for. It can be used for general stupidity but it fits in work situations well. "Nimrod" also works. I'm trying to think of words I've heard my dad say about his coworkers when I was a kid haha. But like others have said, "incompetent" will also really make someone feel bad.

It's not a single word, but similar in origin and meaning is the phrase "can't tell their head from their ass", which means they're very stupid and possibly also clumsy.

ChatGPT suggested ham-fisted and butterfingers along with all thumbs. All of them are kinda playful tho Word compounding is common in English even in formal contexts. I think everyone will understand "ass-handed", use your cultural background for your benefit. Who knows, it can become the next word of year But make sure you are being respectful to those around you. For instance, in Great Britain "arse-handed" will be more appropriate

Inept

I'd call them useless. If I were trying to be even less polite, I might say "they don't do shit"

Someone who is truly useless might be described as a Chocolate Teapot Or Chocolate Fireguard Two things which of course would fail. My mate also used Motorbike Ashtray Again, if you're riding a motorcycle and smoking a little ashtray to tap into would be useless.

Dipshit?

"Dumbass" includes that idea. "Pointy-haired boss" is that for managers specifically, after a character in a comic strip. But mostly I just say "incompetent."

There is a phrase in (British) English that is kind of similar to the literal meaning of your Russian word: you can say someone “doesn’t know their arse from their elbow”. It describes someone who is clueless or ignorant, not necessarily someone who is incompetent at their job.

“You are what the French call: Les incompetents.” - Home Alone Incompetent. You can call them incompetent You can also say as an idiom: “he doesn’t know his ass from a hole in the ground.”

Not quite what you want, but it reminds me of a somewhat old fashioned expression "ne'er do well." The apostrophe stands in for the unpronounced letter V. A ne'er do well might also be called a fuck-up. Someone who gets in their own way, making poor choices. They could be a decent person if they just got some sense into them.

A fuck-up. Useless as tits on a boar.

In America, "Klutz" escaped Yiddish and entered common usage long ago. It means clumsy and stupid. And when confirming some things about "Klutz" I was reminded of "Blockhead". From which I am reminded of "Bonehead", "Knucklehead" and "Numskull". That is Numb + Skull. Implying that the brains are not functioning. "Dumbass" Dumb literally means "Cannot speak" but is commonly used as a synonym for "Stupid". Adding "Ass" automatically makes it more vulgar.

You might call them "someone who couldn't pour water out of a boot with the instructions on the heel," which, while also being a general insult for someone's intelligence, is also used to call someone incompetent. You can also call them an overconfident, defensive, mouth-breathing twat, though that's just a string of insults unlike the expression above.

Nincompoop. Oldie but goodie! Nin-cum-poop (pronunciation) Dumb-ass. Common today. Nimrod. Old but love it. incompetent. (He’s incompetent)

With a phone?

Sluggish, lazy, incompetent, doesn't cut it, and there are plenty other adjectives for someone who is not good at his job, but I guess those words can also be used in other scenarios

Can I say "lacking work ethic"?

Ineptitude.

I've never heard of the term cack-handed before, so I'd assume you're right to assume that it's antiquated. I don't think English really has a single word for what you're looking for. Calling someone "an incompetent" is probably the closest you can get, and the formality of it can sting as well. For adjectives, inept, useless, or (again) incompetent come to mind. Some other words that might be used in a similar way would be noob/newbie (though this implies more inexperience than incompetence) or moron (though this would be used to denote a general lack of intelligence more than anything else). If they stubbornly insist that they are right or that they don't need others help (or worse, that they'd be better off without it) you might call them arrogant. Someone who's very clumsy or who lacks finesse might be called a klutz as well, but that one might be more regional. If you're good with longer phrases, calling someone "dead weight" would imply that you'd be better off without them, or you might say that someone's work "leaves much to be desired" if you want to be more passive-aggressive about it.