Definition of know one's onions in English:

know one's onions

See synonyms for know one's onions

phrase

informal
  • Be very knowledgeable about something.

    • ‘Fred knows his onions about Social Credit’
    • ‘When it comes to real ales and strong continental lagers the landlord really knows his onions, having been lured away from the Fat Cat in Norwich, a pub which wins major awards every single year.’
    • ‘There's room here for a few mavericks, including Canadian and Argentine labels, and - should you not know your onions - the young and friendly staff are happy to pass on their own top tips.’
    • ‘And you need to know your onions when you tackle Beethoven, even before you get to the details of the music.’
    • ‘I'm sure the anthropological brainiacs at the University of Wisconsin, where the research was carried out, know their onions, but I wouldn't count talking about yourself as gossip at all.’
    • ‘As adults, we might know our onions but can't be fagged to do it.’