fossil word

English

Probably from the fact that a fossil was once a living organism, but is still of historical and scientific interest.

Examples
ado, as in without further ado or much ado about nothing fro, as in to and fro yore, as in of yore

fossil word (plural fossil words)

(linguistics) A word that is broadly obsolete but remains in current use because it is contained within an idiom that is still in use.