What is the X in X-mas?
early 14c., "a supplication or prayer, especially to a deity," from Old French peticion "request, petition" (12c., Modern French pétition) and directly from Latin petitionem (nominative petitio) "a blow, thrust, attack, aim; a seeking, searching," in law "a claim, suit," noun of action from past participle stem of petere "to make for, go to; attack, assail; seek, strive after; ask for, beg, beseech, request; fetch; derive; demand, require," from PIE root *pet-, also *pete- "to rush; to fly" (cf. Sanskrit pattram "wing, feather, leaf," patara- "flying, fleeting;" Hittite pittar "wing;" Greek piptein "to fall," potamos "rushing water," pteryx "wing;" Old English feðer "feather;" Latin penna "feather, wing;" Old Church Slavonic pero "feather;" Old Welsh eterin "bird"). Meaning "formal written request to a superior (earthly)" is attested from early 15c.
c.1600, from petition (n.). Related: Petitioned; petitioning.