The tumor is when you have one testicle showing outside of your pants/shorts, you then turn to an audience and ask them " Does this look like a tumor? ". The audience will look down at your ball sack and then they say "what the fuck man" and then you laugh at their stupidity.
Nick: "Hey Rob does this look like the tumor?"
Rob: (looking down) " What the fuck man, why you gotta show me your sack?"
Nick: "Hahahahahahahaha dumbass"