dumbass
1 Even though dumbass [ˈdʌmæ:s] contains the word dumb, it doesn't necessarily refer to someone who doesn't have a lot of intelligence. It’s most often used playfully among people who are relatively close like good friends, siblings, a close coworker, etc., when they have a stupid moment... and let's face it — we all have them now and then 😅 This kind of banter is more common among guys.
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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.
2 Dumbass (and bonehead) is very often used to refer to your own moments of stupidity… When it refers to oneself, it's often expressed with the phrase like a dumbass/bonehead.
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.
3 Naturally, dumbass and bonehead are also used to refer to people in a mean, snarky way when they do something stupid. In other words, the tone of the speaker and their relationship with the object of these words, determine whether it's meant to be offensive or playful. In the snippet below, Nicky is obviously being very critical and offensive of Richard.
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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.