When Rubbish face-plants at the skateboarding rink, Sam confesses he'd trade places with his buddy if he could. Wait, what? Sam knows it hurts to fall off your board, but he's in worse shape after finding out his girl is knocked up. Listen to how he tells it:
He looked at me to see if I was laughing at him, but I wasn't. I meant it. I'd slammed too. I'd never had a slam like this, though. The wheels had come off the trucks, the trucks had come off the deck, and I'd shot twenty feet into the air and gone straight into a brick wall. That's what it felt like, anyway. And there wasn't even a mark on me. (11.67)
Even though Sam isn't physically hurt, we get the idea that he's slammed, or completely thrown off, by the whole pregnancy situation. That's where the book gets its title, Slam, from—it reflects Sam's feeling that the skateboard he's been riding has just exploded, a.k.a. the course of his entire life has changed with one decision. For Sam, everything is derailed, and he feels slammed for pretty much the whole book.