…The phrase, with its imagery of showering an abundance of cash, draws from its roots in the term rainmaker. Recorded in the late 18th-century, a rainmaker is an individual in Native American culture believed to have the power to make it rain (and so aid agricultural bounty). By the end of the following century, rainmaker was applied to professionals would could successfully bring in business.

…As making it rain…is seen as a way of showing off how much money one has to spend indulgently, the expression has become a metaphor for wealth and prosperity. To make it rain, then, has come to mean popularly “to earn lots of money” or “to make something very successful.”