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знать своё дело stresses
gen.	know what one is about (Игорь Primo); know oats; know trade; know one's stuff (быть компетентным специалистом: If someone knows his/her stuff, it means the person is very knowledgeable and skilled in that area – This carpenter really knows his stuff because he has 30 years of experience. Bullfinch); be professional (visitor); be in the zone (cambridge.org Вдохновенная)
amer.	keep eyes on the ball
amer., slang	know how many beans make five; know beans
idiom.	know the ropes (Andrey Truhachev); know one's onions (stackexchange.com); be competent (Abysslooker)
inf.	know what you're doing (SirReal); have the chops (I don’t have great chops as a singer, but I’m determined to get better. Slivas)
Makarov.	know one's trade; know one's business; know one's job; know one's stuff; be on the ball
slang	know one's stuff; know one's business; know one's oil; know one's oats; know one's goulash; know one's bananas; know one's beans; know one's goods; know one's groceries; know one's fruit
знай своё дело
gen.	mind your business

знать своё дело: 27 phrases in 6 subjects

General	12
Makarov	10
Proverb	1
Quotes and aphorisms	1
Slang	2
United States	1