prick up

to get a stiffy, hard-on, boner, etc. when the man's prick goes up.

"dude i got a prick up in the middle of class and had to cover it with my giant notebook"

Prick Up

1. To inexplicably change from a reasonable human being to an incoherent, raving asshat with little or no warning.
i.e. Going from zero to douche in nothing flat.

"We were having a perfectly nice lunch until my coworker decided to prick up when the waiter brought him the unsweet tea he asked for instead of a sweet tea."