Uh, I learned it from my cousins, It’s what we’d say right before jumping into this lake we went to every summer. Basically, it made it harder for people to chicken out when we had this whole chant thing going. What it means, I don’t really know, but that’s the context i’ve used it in.
Part of this phrase was used as the title of the popular Janet Evanovitch novel One For the Money. This use plays on the audience’s familiarity with the phrase. It is used there as a play on words though since it is actually about doing one bounty hunter job literally for the money.