(CNN) -- It's best not to monkey around with Donald Trump -- or maybe that's a bad choice of words.
The billionaire real estate mogul is suing millionaire comedian Bill Maher for $5 million.
Because Maher said -- in jest, as comedians are wont to do -- that he would donate $5 million to a charity if Trump could prove he wasn't the "the spawn of his mother having sex with an orangutan."
"I'm not saying it's true," Maher said told Jay Leno on "The Tonight Show" last month.
"I hope it's not true, but unless he comes up with proof ... I'm willing to offer $5 million to Donald Trump that he can donate to a charity of his choice -- Hair Club for Men; The Institute for Incorrigible Douche-bag-ery. Whatever charity."
Maher picked the figure after Trump, long a skeptic of President Barack Obama's citizenship, offered to give $5 million to the charity of Obama's choice if the president publicly released his college transcripts in addition to his passport records ahead of last year's elections.
The White House never directly responded to the challenge.
Trump says he provided Maher with a New York birth certificate that proved he didn't descend from an ape -- but Maher hasn't paid up.
Trump filed the lawsuit in Los Angeles on Monday.
Included in the court documents are a January 8 letter to Maher, accepting the $5 million charity challenge the day after his appearance on Leno, and a copy of Trump's birth certificate, which lists Fred Trump as his father -- not an orangutan.
It's a free-swinging smackdown by two men used to speaking their minds and not backing down.
Maher, never afraid to ape the object of his pithy humor, was unrelenting on "The Tonight Show" in pushing the simian connection.
"The color of his hair (pauses for laughs) and the color of an orange orangutan is the only two things in nature of the same color," Maher said to another round of nervous chuckles from the audience.
The comedian's reaction to the lawsuit?
"What?? Really? Hahahahahahahahahahahahahahahahahahahahahahahahahaha," he tweeted.
Perhaps, he is waiting for Trump to release his long-form birth certificate.