dumbass

The story

From dumb + ass. It is likely that ¹ass (= donkey), not ²ass (= buttocks), was the original sense within this compound noun (solid compound or open compound), but neither sense is precluded, and many people today may uncritically parse the term in the latter way, even if it is misapprehensive. Apparent analogy with forms such as hard-ass and big-ass would naturally lend support to the second parsing. A similar synergy can be seen in the fact that the terms buttload and assload originated as literally meaning a cartload and the load transportable by a single donkey (respectively), but probably most English speakers today would assume that their derivation involves the buttocks.

First known use, by meaning

• 1959 A stupid or foolish person.