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It's something you say after your male classmates with brains filled with illusions by the fact of your boobs are growing invite you to throw rocks at colorado local cars and SUVs.
Stan: Ah, hey, Bebe.
Bebe: Yeah?
Stan: Uh, we're gonna throw rocks at cars later on, and we thought maybe you'd like to join us.
Bebe: Really? I've never done that before.
Butters: Wuuhh, it's really fun.You toss these little rocks at cars, and if the driver gets angry, you blame me.
Kyle: Yeah, it's cool.
Bebe: Why sure, that sounds hellafun.
The boys: Hahahahahaha...
Kyle: Yeah.
Stan: Hellafun
Cartman: Right, heh.
Clyde: Wow, hellafun. That's awesome.
Token: I'm gonna start saying that now.
by zhd December 21, 2008
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Aug 11 Word of the Day
A phrase to describe someone who is cognitively degenerating. Synonym of "going off the deep end". Can have varying degrees of severity.

Reference to the song "Hey You" by Pink Floyd. The line "and the worms ate into his brain" makes no sense in an otherwise linear and literal narration throughout the lyrics.
"My boyfriend has a total case of brain worms. He told me the cat was bugged so the Feds could listen in on us having sex..."

or

"You totally stumbled over that entire sentence. Can't speak English all of a sudden? What, do you have brain worms?"
by _Jez_ October 03, 2009
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