If you're familiar with the term "Cock Block," you know that it means one dude messed up another dude's game with the ladies. Well, this happens to girls too. And we (most of us) don't have cocks.
If you've ever been obviously checking out a guy and then your friend decides to "claim" him by walking up to him and touching his polo shirt and exclaiming, "I'm soooo drunk right now omg!"- you've been clam jammed.
If you've ever been hitting it off with a guy and then you tell him his pong game is weak, you've been clam jammed.. by yourself, I might add.