Acronym for "Random Class Boner".
The often embarrassing occurrence, of a prepubescent male being afflicted with a spontaneous, uncontrollable erection.
Applies strictly to classroom situations, but has been adapted by social decree, to be used to describe any embarrassing erection regardless of setting.
Aww man. Double geography with Ms. Webster. Gonna be getting serious RCB's now for the next 80 minutes.