How do you say "adieu" in French?

‘Adieu’ literally means ‘to God’. Adieu is a loan word which has crossed over to the English language, but English speakers tend  to say it so that the ‘-dieu’ sounds like ‘dew’, which is perfectly acceptable in English, but not French. You use it when you mean to say goodbye or farewell  for a long time, or forever, as opposed to ‘see you soon’.  See also ‘au revoir‘.