Experienced, informed, or knowledgeable; well-versed in something (borrowed from French 'au fait').
From French 'au fait,' literally meaning 'to the fact' or 'to the point,' where 'au' means 'to the' and 'fait' means 'fact.' Adopted into English to describe someone well-informed.
English loves borrowing from French to sound sophisticated—'au fait' lets you say someone is 'in the know' in just two words, which is why it stuck around in English!
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