Green’s Dictionary of Slang

harsh v.

[harsh adj.]

1. (US campus) to mistreat, to be very unfair towards; thus harsh me out! that’s very unfair!

[US]Eble Campus Sl. Mar.
[US](con. 1970s) G. Pelecanos King Suckerman (1998) 94: You gonna let your lady harsh me out like that, bro?

2. (US) to ruin, to damage.

[US]Eble Campus Sl. Apr. 4: harsh someone’s mellow – diminish someone’s happy mood.
[UK]N. Gaiman Anansi Boys 83: Brother of mine, you are harshing a potential mellow here.
[UK]D. O’Donnell Locked Ward (2013) 138: It was all an Establishment plot by the pigs to hassle him and do the dirty on all heads and hippies by constantly harshing their mellow.
[US]T. Pluck ‘Six Feet Under God’ in Life During Wartime (2018) 192: ‘I got tired of mean people like you harshing my mellow’.
[UK]‘Aidan Truhen’ Seven Demons 45: ‘[I]t is harshing my obscene wealth buzz’.
[UK]M. Herron Secret Hours 389: ‘Because letting an amoral, self-serving, ruthless bastard through the door would harsh the Park’s mellow’.

In phrases

harsh on (v.)

(US campus) to criticize, to belittle.

[US]Eble Campus Sl. Spring.
[US]Eble Sl. and Sociability 30: Athough English has dozens of particles available for word building, almost all of the examples in college slang use out, on, off and up: [...] harsh on ‘criticize, belittle’.