While a few will swear they remember it from high school in the 70's (and in fact characters on The 70's Show frequently used identical or similar insults, reinforcing that notion), it turns out the answer is actually really simple:
Mike Judge just made it up.
So there’s dill weed the spice of course, and obviously I didn’t invent that name. But if you’re calling somebody that as an insult, I’m pretty sure it came from the show. Here’s how it happened: when I was in junior high in the 70s people would call each other dick weed. It was a pretty common insult. I tried to do it in ‘Beavis and Butt-Head’ and standards and practices at MTV wouldn’t let me say it. They wouldn’t let me have them call each other ‘dildo’ either.
So one day when I was recording, I had Butt-Head call Beavis ‘dill weed’ and it kind of got a laugh, and it cleared with the network censors, so I just kind of kept doing it. The rest is history I guess.
("Nimrod" is a biblical name but, as far as I've been able to determine, its use as an insult seems to originate with the Bugs Bunny short "Rabbit Every Monday":
Subjectively 'dumbass' is something I recall hearing way earlier with far more ubiquity. Somewhat more objectively, it seems etymologically older, as do most insults with ass in them. "Stupid donkey" or "Stupid idiot" (from the older usage of 'ass' as an insult) makes more sense than "Stupid butt" (unless your age is measured in single digits, in which case any version's all good as well as intrinsically funny).
EDIT: A quick google search shows shows a reference dating to '59. EDIT 2: Merriam-Webster pegs the first recorded use in '58. Collins says the hyphenated version, 'dumb-ass' dates back to the 18th century.
EDIT 3: I just noticed the google entry for Urban Dictionary's version is "Someone who looks up the word "dumbass" in a dictionary." xD
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