What is piss-gasm?

A piss-gasm is the orgasm a guy has when he's taking a piss and has a cumgasm because his bladder is pressing on his prostate gland.

I was taking a piss and had a piss-gasm!

piss-gasm - meme gif

The titillating sensations one feels whilst urinating. Usually accompanied by a yell or moan, of some sort.

"Oh, Dear God. I've been holding this one all day. Ohhhhh, yea, I'm having a piss-gasm."