Why is the ninth month called September?
late 14c., from Anglo-French defeter, from Old French desfait, past participle of desfaire "to undo," from Vulgar Latin *diffacere "undo, destroy," from Latin dis- "un-, not" (see dis-) + facere "to do, perform" (see factitious). Original sense was of "bring ruination, cause destruction." Military sense of "conquer" is c.1600. Related: Defeated; defeating.
1590s, from defeat (v.).