(Verb) Join up, become part of a group, generally a group that is going to go do something mischievous and/or unlawful. In an MMO, it means to become part of a questing/PvE party or PvP party.
Not that this is completely different from throwing in the towel.
Dan: Bob and I are gonna go somke some weed and egg a house, you coming?
Frank: Sure, I'll thrown in for that shit any day.
Lamerrr: LF2M BRD
Dissonance: BRD? I'll throw in, as long as you don't ninja loot anything.
(Verb) Conststantly getting a frame rate that isn't unplayably low, but low enough to cause considerable skips in video, generally around 10fps. The gap in frames make the game seem as if it were a flipbook, ergo the term.
There was a big fight outside Orgrimmar last night and I really wanted to throw down, but I kept flipbooking and got my ass royally pwnt.