male ejaculation in an adult film. adult film stars use to get paid by the number of times they ejaculated, hence "money" and the "shot" speaks for itself.
10,000 moneyshots in one session will make you a rich man.
The money shot is derived from the pornographic film industry. When the main actor is unable to ejaculate a second steps in with the sole purpose of ejaculating on the woman (this is why you so often see very little of the man in the final shot, so you can't tell its a different actor.)
Called the money shot because the second actor traditionally gets half the money for doing very little