Douglas Harper's Etymology Dictionary
Wiktionary
a. stupid, foolish n. (context US vulgar slang English) A stupid or foolish person.
Wikipedia
Usage examples of "dumbass".
Police knew without being told to take a close look at anyone who might have come from a country east of Israel, and if some dumbass lawyers called that ethnic profiling, then to hell with him.
I’ll be happy searching because I know it’s not pointless, I won’t suffer like those dumbasses that wound up trapped.
These dumbasses get it into their heads to get a hummer from their fungdu, and then are understandably surprised to discover that their pet thinks it's being fed a live hot dog.
That bunch of food-coupon-grabbing good-for-nothing welfare dumbasses get whatever they can from the US, the French and the Brits before the dung hits the fan, we get the hell out and they go back to incest and alcoholism.
I gotta give you credit, Maggot: You may be a chinless, gritless, dumbass Canaanite, but sometimes you're almost as bright as a real Carthaginian.