When someone of the male gender stops to pee in public or is at a urinal, another male sneaks up behind him and urinates on his butt. This is the act of pee butt.
"Dude, why are your pants all wet, did you piss yourself?"
"No dude, it's not actually my piss! Some asshole pulled over after me on the freeway and gave me a pee butt!"
Pee shyness is when someone cannot pee with others in the immediate area. This is common in women, who commonly go to the bathroom in herds. Sometimes this is relieved by everyone in the immediate vicinity turning their backs to the person on the "throne", running water, or making other distracting noises.
Char asked all the girls in the bathroom to make noise because she had pee shyness.