It is derived from the word money when used in the same way to describe something of a high standard. Example: Those shoes are money, you are going to get all the ladies.
Bank or Bankass is used when something is cooler than the word money can convey. This is because money in the bank has already been laundered and is earning interest.
"Bank" is the shortened version of "Bankass" and has the same meaning.
2.) Hot Girl A: "Hey have you seen Zac, he looks cool"
Hot Gril B: "Yea that guy is so bankass, yum!"