Nicki Minaj -- Bitchy to Mariah Carey For Years

Nicki Minaj was supremely disrespectful to Mariah Carey 2 years before she exploded on the set of "American Idol" this week ... TMZ has learned.

Nicki and Mariah teamed up in 2010 for a music video -- "Up Out My Face." Nicki wasn't nearly as big at the time -- Mariah was, and we're told she felt like she was helping launch Nicki's career.

B. Scott, who's big on the Internet, appeared in the video. He tells TMZ ... "Mariah went out of her way to be nice to Nicki and make her comfortable." But B. Scott says Nicki was a raving bitch, "making faces at her [Mariah] laced with rude comments under her breath."

B. Scott says he blew the whistle on Nicki by giving Mariah the low down. B. Scott says Mariah decided not to confront Nicki and acted as if nothing had happened.

A source close to Nicki tells TMZ ... B. Scott is a liar because, "He works as Mariah's stunt double and of course he would say things like that. He's worked for her for years."

B. Scott says he stayed in touch with Mariah and spoke with her Thursday night. Mariah says she wants to bury the hatchet with Nicki on "A.I." and focus on the contestants.