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Someone or something that harshes one's mellow.

A person, place, thing or action that destroys one's cool and laid back mood, at least temporarily.

Can be used interchaneably with the corresponding phrase 'harshed my mellow'.

Usually stated as, "Doooooode....that's a mellowharsher."
Bob lied to me yesterday, and when I found it, it really harshed my mellow. It was a mellowharsher.
by mommadavis June 14, 2011
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