The spiderman is something that often happens to guys that go to prison on their first night in the shower. All the other guys beat up the "fresh fish" and push him to the ground in the corner of the shower then they form a circle around him and beat off till they cum in their hand and sling it at the new guy that all beat up and crying like a little girl. As they throw the cum at him they yell "Go Web!". Its a way of showing the new guy that from that minute on he's somebody's bitch.
Damn, look over there, that guy's gettin spiderman'ed! They pushed him to the floor, beat off in their hands and threw the nut all over him. He's somebody's bitch now..... sad.