Be tired out, exhausted, as in I can't walk another step; I'm all in.
[ ; second half of 1800s
In card games, especially poker, be out of money, as in I'm finished for the night; I'm all in. It refers to having put all of one's money in the pot. In his historical dictionary of slang, J.E. Lighter suggests that the gambling usage, first recorded in 1907, may be the source of the first sense.