Know one's onions:
Well acquainted with a given topic.
Knowledgeable, experienced.
Examples:
Mr. Roberts knows his onions, all right.
The Columbus statement declares that the league is ready to give the Donahey boom every aid and comfort. Of course! Why not? Governor Donahey knows his onions…
I know my stuff. I can do my job. She can't handle the assignment. She doesn't know her onions.
That car salesman certainly knew his onions, didn't he?
Definitions from the English-Bulgarian Dictionary:
Onion ['ʌnjən]
Onion, bulb onion.
sl. Head, brain.
mil. sl. Incendiary rocket.
Know [nəu] – to know, to be acquainted with, to be familiar with.
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