Dolly Parton -- My Halloween Costume is Gonna Be TRASHY!!!

Dolly Parton is gonna be a NAUGHTY girl this Halloween ... and she's not afraid to admit it.

Parton was having dinner at Matteo's in L.A. last night ... when she told us she had picked up a sexy costume at the famous Trashy Lingerie shop earlier this week ... and it sounds like it's gonna be AWESOME.

"We have fun at Halloween," Parton told us ... and as she grabbed her legendary boobs, she added, "I have two pumpkins ... that's all I care about."

God we love you.