學個片語：＂To know your onions＂

If someone says you know your onions, it means you are experienced in something or you know a lot about a particular subject. So really, it means you're clever! But why 'onions'? It is a strange term and some people say it comes from a man called S.G. Onions who made fake coins to help teach children about money. If they learnt about money they would know their onions. So knowing your onions means knowing a lot about something. Another way of saying it, is that you really know your stuff.

Samples

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If you ever need someone to fix your computer, ask Bob, he really knows his onions!

Look, you got full marks in the maths test – you really do know your onions.

That tour guide really knew her onions, didn't she?

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＂To know your onions＂指某人精明幹練或精通擅長某事！乍看之下意思跟詞合不太起來，有這麼一說是，叫作S.G. Onions的人用一套代幣做為教具教會孩子關於錢的概念跟使用，所以用know your onions代表學到了且熟練精通！(-_-)這種詭異而牽強附會的詞在1920s的美國層出不窮，上回講的"the bee's knees"也是同時期的產物！在美國俚語中，onions也可指人的腦袋！除了＂know your onions＂外，意思等同"know your stuff＂，也是指精通其業務！

From BBC learning English