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.
