know one's onions

present singular third person knows one's onions · participle present knowing one's onions · past knew one's onions · participle past known one's onions · alternative know one's onion

1. informalSynonym of know one's stuff: to be knowledgeable about a given subject or task.

   Joe really knows his onions when it comes to cooking.

   1 more example

   Sam Tyler: Well, our first priority is to make sure the thieves don't strike again. It'll be tricky, 'cause these sorts of gangs rarely have just one target on the go. They've usually researched half a dozen possibilities, waiting for the most opportune circumstances on any of them. / Harry Woolf: He knows his onions, this lad, eh? / Gene Hunt: Oh, he's a right little smart-arse. So, what do you suggest, Buddha?

   2007 February 20, Chris Chibnall, Life on Mars, season 2, episode 2

   Synonyms: know one's stuff