What is the X in X-mas?
early 13c., from Old French consentir (12c.) "agree, comply," from Latin consentire "feel together," from com- "with" (see com-) + sentire "to feel" (see sense (n.)). "Feeling together," hence, "agreeing, giving permission," apparently a sense evolution that took place in French before the word reached English. Related: Consented; consenting.
c.1300, "approval," also "agreement in sentiment, harmony," from Old French consente, from consentir (see consent (v.)). Age of consent is attested from 1809.