'au fait' originates from French, specifically the phrase 'au fait', where 'au' meant 'to the' and 'fait' (from 'faire') meant 'done' or 'fact'.

'au fait' was borrowed into English from French; it entered English usage as a borrowed phrase and has also appeared respelled as 'aufait' or hyphenated as 'au-fait' in English texts.

Initially in French it conveyed the sense 'to the fact' or 'done/ready,' but in English it evolved into the current meaning of 'well-informed' or 'familiar with'.