French Montana -- I Slacked Off With Khloe ... But Now I've Got Some Serious Abs

French Montana admits he packed on a few lbs while he was dating Khloe Kardashian ... but now that they're history, he's transformed his oversized gut into rock-hard abs.

French saw Khloe turn into a fitness machine after their breakup in early September, and he followed suit ... hitting the weights with producer Mally Mall at CrossFit in Sherman Oaks.

We're told Khloe's appearance in workout videos and her obvious weight loss the last couple months has been French's inspiration.

Another motivation ... French also has a new album set to drop in December.

Rob Kardashian, your move.