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1.
Bringing down my good mood.
I didn't like her.. she was really harshing my mellow, man.
by Julia May 07, 2005
 
2.
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
 
3.
A stupid idiom that Mackenzie wants to believe is real, when it really isn't.

This isn't a real phrase and has been completely made up.
Mackenzie:
What does "harshing my mellow" mean?

Anyone who isn't an ignoramus:

It doesn't mean anything, because you just made that phrase up and it doesn't exist.
by American Dictionary of Idioms. April 10, 2011