In (poker)
1) How many chips a player has bought altogether. "How
much you in?" might be an attempt by another player to
find out whether that large stack of chips you have is winnings
or all your own money. Also, in for.
2) Taking part in a pot. "You in?" means "Are
you partaking in this sporting venture?"
3) Having anted. In this context, "You in?" means
"Did you ante?" (and implies that you didn't).
