To do something that destroys one's plans; particularly, to ruin someone's party night by asking them to babysit, borrowing their car, or otherwise render them incapable of attending said function.
Dude, mom asked me to babysit tonight, she's totally harshing my gig!
(harsh a gig)
When one inhales too much marijuana smoke, one may find oneself coughing up his lungs. The quality of the weed may make a harsh toke, but it's is usally down to error of judgement by the toker. If he drags too much on a joint, he may experience a 'harsh toke,' especially if he takes it straight down. However a harsh toke usually occurs when toking from a bong, as it is sometimes hard to judge just how much smoke is in the chamber.
Guy 1: *Cough* *cough* *cough* *cough* *cough* *gak* *cough* *cough* *cough* "Fuck dude that was oneharsh toke!!"
Guy 2: "Dude, that toke looked harsh as."
Stoner (marijuana smoking) origin. When an incident occurs that causes one disappointment after a period of happiness or contentment. Akin to "bursting a bubble."
Also: Harsh(es)(ed)(ing) my buzz
I was really excited about my new play station, but when the power went out it totally harshed my buzz.