Chyna -- Going Hardcore with Joey Buttafuoco ... in the Gym

Chyna is sticking it to Joey Buttafuoco in a famous whorehouse -- but not for a porno ... she's training him for an upcoming boxing match.

TMZ has learned Buttafuoco and Chyna -- the wrestler-turned porn star -- are working out at The Bunny Ranch in Carson City, Nevada. We're told owner Dennis Hof is giving them "round the clock use of the accommodations, gym and personal chef." It's unclear what the "accommodations" include.

Back in 2002, Chyna and JB fought a celebrity boxing match -- Joey squeaked out a victory -- and next month he's set to fight his ex-mistress Amy Fisher's husband.