to be a killjoy. to ruin someone's happiness, whether they are drunk, or just really happy, with sad news or drama.
mellow, in a good mood, then someone tells you something saddening. u harshed the mellow.

"dude. your house is on fire."
"damn. you totally harsh my mellow."
by kiley hamre November 7, 2007
1. To annoy someone and bring them down whilst they're in a good mood
2. To annoy someone and bring them down whilst they're high
1: "This guy's annoying the fuck outta me, he's totally harsh my mellow!"
2: "Like duuude, this guy just came along and was screaming at me and dude, whoa dude HE TOTALLY HARSH MY MELLOW man!"
by LordEuronymous May 27, 2012
Bringing down my good mood.
I didn't like her.. she was really harshing my mellow, man.
by Julia May 7, 2005
when you're peacefully blazed and something happens to ruin your high..
"Dude got baked and was screaming and shit. He completely harshed my mellow man."
by PsychoDice January 14, 2004
When someone takes away from your high, or tries to be cool by fucking with you when you are buzzed. What happens when a nancy (see Nancy) brings down your elated feelings while high.
Dude, you're really harshin' my mellow.
by Duke March 13, 2005