The phrase has to do with reliability. You can wager money on what will happen, and if you have inside information and you're 100% certain your bet is
right, then your pay-off is assured; you might as
well call it "money in the
bank."
You can also use the phrase to describe someone who is very consistent in their behavior, because if you bet on him or her, you know you'll win. It's usually used in a positive sense to describe someone who always delivers and brings joy or victory. A synonymous phrase would be "reliably
good."
The
slang use of the term "money," as in "you are so money right now" is presumably derived from "money in the
bank." It means not just excellence, but excellence demonstrated on repeated occasions, establishing a pattern of reliability. It can be used to compliment someone for just
one instance of excellence, but it still implies that this
one instance reflects on the excellence of the person's larger character, that they would be expected to be excellent again.
Yankee fan 1: Do you think Mariano can close this thing out?
Yankee fan 2: Are you kidding? He hasn't allowed an earned run in 2 years. Money in the
bank,
baby.
Indians fan (weeping): I am holier than thou in my suffering.
Yankee fan 1: Sorry, I couldn't hear you, Indians fan. The
wheel of my victory chariot must have been lodged in your windpipe.
