What is the X in X-mas?
1590s, "acting as if benumbed," alteration of Middle English clumsid "numb with cold" (14c.), past participle of clumsen "to benumb, stiffen or paralyze with cold or fear," from a Scandinavian source akin to Old Norse klumsa "make speechless, palsy; prevent from speaking," intensive of kluma "to make motionless." For insertion of -s-, cf. flimsy.
Not in general use until 18c., with senses "manifesting awkwardness; so made as to be unwieldy." Related: Clumsily; clumsiness. Cf. Swedish dialectal klummsen "benumbed with cold," Norwegian klumsad (past participle) "speechless, palsied by a spasm or by fear or witchery;" German verklammen "grow stiff or numb with cold." Also cf. clumse (n.) "a stupid fellow."