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Note that bonehead can be used in all the same cases below, with the same meanings.
Jamie, opening a pack of ramen noodles...
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Jamie: Am I supposed to eat these right out of the box?
Nicky: No, dumbass, it’s noodles — you have to boil them.
Nicky: Hey, can I invite my friend, Billy, to your party tonight?
Ron: Go for it.
Nicky: Ok, cool, I’m gonna send him your address. [sends text]
Ron: You sent it to me, dumbass.
Nicky: Oh! My bad.
What a dumbass! I just went to brush my teeth, and like a bonehead, I grabbed the wrong tube and put sunscreen on my toothbrush.
It's also considered a genuinely negative when used as an adjective, spelled dumb-ass, as in Put out that cigarette — here comes your dumb-ass neighbor or Stop playing that dumb-ass video game and help me with this math assignment. Here, it doesn't refer to anyone's intelligence, just to someone or something that's an "inconvenience" to you at the moment. In this usage, we also use stupid-ass. This is simply a combination of these adjectives with the suffix -ass, explained here. However, dumbass as a noun refers exclusively to people and is written as one word.
Alex: They just put a new microwave in the break room. So the first one to use it is Richard from Accounting... he puts in a piece of lasagna wrapped in tin foil and it was like New Year's fireworks in the microwave.
Nicky: What a dumbass! Even my 5-year-old nephew knows not to put metal in a microwave.