to give up due to exhaustion. Originally a term referring to a tired horse that begins to run less aggressively, backing off on the "pull" a rider normally feels on the reins from an eager horse. Also used as a generic term for an exhausted horse.
"Yes, George, come in, come in. George, word up's you've been cracking under the pressure. Can't cope, can't stand the heat. Spit the bit." - George Steinbrenner, speaking to George Costanza on "Seinfeld".