The term “Rain Man” originally came from the 1988 movie Rain Man, starring Dustin Hoffman and Tom Cruise. Since then, it has been used to describe people with random comments or shady businesses and in rap lyrics in a totally different context.
Rain Man in Film
In the 1988 film "Rain Man," Dustin Hoffman plays a man named Raymond who has autism and savant syndrome. He has developmental disorders but has an amazing memory. In this movie, Raymond is called “Rain Man” because that is the name his younger brother (Tom Cruise) gave him as a child when he couldn’t pronounce “Raymond.”
Barry Morrow, the writer of "Rain Man," chose the title of the movie by flipping through a book of names to find which ones sounded best when mispronounced. He narrowed it down to Raymond (“Rain Man”) and Norman (“No Man”) before asking his children, who chose Raymond.
Rain Man as a Description
“Rain man” can be used to describe someone who makes random or odd statements with no provocation. For instance, if you were watching a show about how stamps are made and someone said, "I like turtles," he might be considered a "rain man."
Rain Man as a Shady Person
"Rain man" can refer to a person who is selling something questionable. If you give the "rain man" your money and you get the product or service in return, you have gambled but ultimately received what you paid for. There is an equal chance that you could give the "rain man" your money and he will not give you the product or service in question.
Rain Man in Popular Culture
In pop culture, artists like Lil Wayne, Eminem and Jamie Foxx refer to “rain men.” The song “Got Money” by Lil Wayne featuring T-Pain says, “Everybody say Mr. Rain Man can we have a rainy day? Bring an umbrella, please bring an umbrella.” “Rain Man” by Ya Boy featuring Lil Wayne says, “Imma Rain Man, I make it thunderstorm, I make it Hurricane.” In this context, “Rain Man” means someone who carries a lot of money and “makes it rain” by throwing cash at strippers.