know one's onions

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know one's onions (third-person singular simple present knows one's onions, present participle knowing one's onions, simple past knew one's onions, past participle known one's onions)

1. (informal) Synonym of know one's stuff, to be knowledgeable about a given subject or task.

   Joe really knows his onions when it comes to cooking.

   • 2007 February 20, Chris Chibnall, Life on Mars, Season 2, Episode 2:

     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?

• “know one's onions”, in Collins English Dictionary.