Stories We Like: A Guide to the Comma
late 14c., "to make an answer," from Old French replier "to reply, turn back," from Late Latin replicare "to reply, repeat," in classical Latin "fold back, fold over, bend back," from re- "back, again" (see re-) + plicare "to fold" (see ply (v.1)). Latin literal sense of "to fold back" is attested from mid-15c. in English but is not now used. Modern French répliquer (Middle French replier) is directly from Late Latin. Related: Replied; replying.
1550s, from reply (v.).