Douglas Harper's Etymology Dictionary
n. 1 support; agreement; approval; blessing (in a secular sense). 2 (context poker English) A tournament where a player must purchase all of his or her chips before the tournament starts. 3 (context poker English) The amount that a player buy in for
Usage examples of "buy-in".
You've been getting penalized like a player in a tournament who oversleeps and automatically gets all the antes and blind bets deducted from his absentee buy-in, and those involuntary bets have—have cost you, horribly.