know (your) onions

British & Australian, humorous

to know a lot about a particular subject.

That car salesman certainly knew his onions, didn't he?

• the nitty-gritty
• the nouveau riche
• the nuts and bolts
• the odd man out
• the odd one out
• the Old Bill
• the old-boy network
• the old country
• the oldest profession
• the oldest profession in the world
• the oldest trick in the book
• the old guard
• the old school tie
• the only game in town
• the operative word
• the order of the day
• the other day
• the other side of the coin
• the other side of the tracks
• the pants off
• the pants off someone
• the parting of the ways
• the party is over
• the path of least resistance
• the patience of a saint