Goodbye, or a farewell expression, especially used in formal or dramatic situations.
From French 'adieu,' which comes from 'à Dieu' (to God). Medieval travelers would literally commend each other to God's care when parting, as travel was dangerous and mortality uncertain.
When you say 'adieu' you're literally saying 'to God'—it reveals how much religion was woven into everyday medieval language, and why saying goodbye was practically a prayer that you'd meet again alive!
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