mid-14c., expounen, expounden, "to explain or comment on, to reveal the meaning" (of Scripture, etc.), from Old French espondre "expound (on), set forth, explain," from Latin exponere "put forth, expose, exhibit; set on shore, disembark; offer, leave exposed, reveal, publish," from ex "forth" (see ex-) + ponere "to put, place" (see position (n.)); with unetymological -d developing in French (compare sound (n.1)). The usual Middle English form was expoune. General (non-theological) sense of "set forth, reveal, describe or tell" is from late 14c. Related: Expounded; expounding.
'In Englissh,' quod Pacience, 'it is wel hard, wel to expounen, ac somdeel I shal seyen it, by so thow understonde.' ["Piers Plowman," late 14c.]