"dejected, dispirited," 1580s, creast falne, it has the form of a past-participle adjective, but the verb crestfall is recorded only from 1610s, in reference to diseased horses, and is rare. It's possible that the image behind this use of the word is not having the crest fallen, as a defeated cock does, but horses. Crest-risen "proud, lusty" is from 1610s.
