The homosexual tendency for an inmate to glance at another while showering in jail or prison. A shower shark will usually stand casually on the upper tier pretending to chill or talk, then when the opportunity strikes, glance at their victim in the lower level shower. Shower Sharks also walk by inmates showering and catch a quick glance.
"Despite what people may say, I am not a Shower Shark!" "Yo...check that dude Shower Sharkin' it!"