The phrase may derive from the act of passing around a hat, into which contributors would "throw in" some money.
Not: "There's orphans starving in Haiti? I'll throw in for that!" Situation too formal or grave.
Not that this is completely different from throwing in the towel.
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.
Geezie said "Man, you just throwin me, ya'll knows where she's at" and when I findz her, I'm-a pop a cap in her azz.