Possibly originating with gambling games such as Texas Hold 'em style poker, where the maximum 'raise' is to bet your full stack of chips.
Gary Coleman: "I can't, bitch...I'm all in."
If you have a very good hand, go all-in to win the maximum number of chips or to scare off mediocre hands so they won't catch the cards they need to beat yours. If you have a bad hand, you can bluff by going all-in and hope everyone folds.
Player2: I fold.
Me: I call ya.
Player1: Full House ;)
Me: Four of a kind.
Player1: ... Which one's better?
Player2: I'm pretty sure Mike's hand beats...
Player1: Who the fuck's talking to you, old man?
Me: Well, it's been a pleasure, Player1.
Player1: No, wait, I still have 5 bucks left... c'mon, let's play... c'mon, man! Don't go... don't... Yo... Player2, wanna play some more?.
Player2: Screw ya.
All in means you don't stop for Sundays.
All in means nobody can talk you out of it.
(from New York Times online, October 17, 2011):
Mr. Immelt’s remarks took on the tone of a halftime pep talk. He said that with a clearer regulatory structure, an increased export base and an “all-in” business climate, the United States would be able to compete on a global front.
---Note that the Times used the term 'all in' with a hyphen separating the two words, which is customary when such a term is used as a single adjective. (Compare: "Frank is just flat-out broke".) Also note that the Times put slightly distancing quotation marks around the phrase in the above Immelt citation. This probably means that the Times writer recognized the phrase as a colloquialism, not yet fully acceptable standard written English, in this extended (non-poker) usage. Some grammarians (cf. Strunk and White, THE ELEMENTS OF STYLE), object to ironic or distancing quotation marks on the theory that if a term or phrase is known to most readers, introduction or contexting is not necessary. Most likely, though, the New York Times' elaborate style sheet does not forbid such use.