Jamie Foxx & 2 Chainz -- Major Party Buzz ... New Song Copies Old Song

6:53 P.M. PDT -- According to a rep, "DJ Mustard did not grant any rights to a third party to exploit the recording, "Party Ain't A Party, " and, accordingly, this claim against DJ Mustard has absolutely no merit."

Jamie Foxx callously crushed the dreams of an up-and-coming singer ... by jacking his song -- "Party Ain't a Party" -- so Foxx could selfishly resurrect his own music career, according to a new lawsuit.

Foxx released the song -- featuring 2 Chainz -- as a single this week ... "after being absent from the music scene for over four (4) years," according to court docs obtained by TMZ.

Just one problem -- it's the EXACT same song recorded by a young artist named J Rand a year earlier! Seriously, listen ... the ONLY difference between the two tracks are the singers.

Nontra Records says the rip-off is blatant and shameless -- because J Rand's version and Foxx's version were both produced by the same guy -- DJ Mustard.

Nontra's suing all 3 guys -- Foxx, 2 Chainz and DJ Mustard.