Yours, Etc.: Origins and Uses of 8 Sign-Offs
c.1400, "be successful; be efficacious," from Old French prevaleir (Modern French prévaloir) and directly from Latin praevalere "be stronger, have greater power," from prae "before" (see pre-) + valere "have power, be strong" (see valiant). Spelling in English perhaps influenced by avail. Related: Prevailed; prevailing.