NFL star Marcedes Lewis -- MMA BEAST-IN-TRAINING ... Check Out My Arm-Bar Skills!

You don't wanna screw with Jacksonville Jaguars tight end Marcedes Lewis at the bottom of a pile ... 'cause dude's training to SNAP ARMS ... and he's getting pretty good at it.

The 6'6", 272-pound giant has been spending his off-season working on his MMA skills -- and training with NFL insider/MMA junkie Jay Glazer out in L.A.

Well, it looks like the training's been paying off ... 'cause Lewis hit the mat for an arm-bar training session recently ... and was pretty impressive ... even forcing his partner to tap out.