What is the X in X-mas?
Old English oferdon "to do too much," from ofer (see over) + don (see do (v.)). Common Germanic (cf. Old High German ubartuan). Meaning "to overtax, exhaust" (especially in phrase to overdo it) is attested from 1817. Of food, "to cook too long," first recorded 1680s (in past participle adjective overdone).