A Pi Lam slam is in reference to the fraternity: Pi Lambda Phi.
It is a noun used to refer to a girl that has slept with AT LEAST 4 members of Pi Lambda Phi. Some will stop at 3 just to avoid the bestowment of the term, in which case they may be referred to as "one to go".
Brodysseus: Hey man, how far off is Cindy?
Brochacho: One more to go as of last night.
Brodysseus: Inching close to the Pi Lam Slam, haha.
When multiple Pilams fuck the living hell outta a bitch, then more Pilams fuck the same bitch, and then yet more Pilams fuck the same exact bitch.
Guy #1 - Hey man, I fucked this total skank last night
Guy #2 - Holy Shit! The skank from Zeta's party? Me too!
Guys #3, 4, 5, 6, 7 & 8 (at the same time) - ME TOO!!!!
All Guys - HELL YEAH THE BITCH GOT A PILAM SLAM DONE ON HER!!!!