Origin: 1590–1600; clums benumbed with cold (now obs.) +
-y 1 ; akin to ME
clumsen to be stiff with cold, dial. Sw
klumsig benumbed, awkward,
klums numbskull, ON
klumsa lockjaw.
See clam 2 
Related forms: clum⋅si⋅ly, adverb
clum⋅si⋅ness, noun
Synonyms:
1. ungraceful, ungainly, lumbering, lubberly. 2. unhandy, unskillful, maladroit, inexpert, bungling, bumbling, heavy-handed, inept.
Antonyms:
2. adroit, skillful.