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1.
To enter all of your poker chips on one hand.
When he saw he had a royal flush, Muhammed Ali went all in.
by Brian/Larry/Ziggy/Morpheus June 24, 2004
 
2.
To be totally committed to something.

Possibly originating with gambling games such as Texas Hold 'em style poker, where the maximum 'raise' is to bet your full stack of chips.
If we're gonna do it, let's do it. I'm all in.
by Honor June 23, 2004
 
3.
When not used in a poker game context, "all in" means that one thing is completely inside something else. No more of said object will be able to enter after this point. Often used in sexual situations.
Linda Tripp: "Go deeper baby, I can't even feel it."
Gary Coleman: "I can't, bitch...I'm all in."
by Nick D July 28, 2004
 
4.
In no-limit poker, to bet all of your chips as a sign of total confidence in your hand.

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.
I went all-in on thirty miles and that river rat caught a runner-runner flush.
by Coell November 10, 2005
 
5.
To play all of your chips in a game of poker.
Player1: I'm all in.

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.
by ChaozMike August 31, 2006
 
6.
Originally and still a poker metatphor, 'all in' has also come to mean a situation whose subject is unreservedly involved, without qualification. Fully committed. In this sense the term "all in" is almost the same as its denotative opposite, "all out," as in all-out warfare.
.

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.
by al-in-chgo October 17, 2011
 
7.
1. Investing all of your holdings at once, as in a single hand of poker.
2. Extremely tired, or at wit's end. (slightly archaic)
1. You went all in on a pair of nines?
2. I can't talk about this right now, I'm all in.
by mycrows March 08, 2010