1590s, "acting or moving as if benumbed," alteration of Middle English clumsid "numb with cold" (14c.), past participle of clumsen, clomsen "to benumb, stiffen or paralyze with cold or fear" (early 14c.), also "become numb or stiff, as with cold" (late 14c.), which is 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-, see flimsy.
Not in general use until 18c., with senses "manifesting awkwardness; so made as to be unwieldy." Related: Clumsily; clumsiness. Also compare Swedish dialectal klumsen (adj.) "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 compare clumse (n.) "a stupid fellow."