Tim Tebow: Terrible Lip Syncer ... Crazy Insane Physique

Tim Tebow can't sing.

He can't even pretend to sing.

But damn ... dude is jacked.

Tim is a contender on Thursday's "Lip Sync Battle" -- and one of his songs is "Eye of the Tiger" by Survivor.

Right from the beginning, Tim abandons the faux vocal part of the show ... instead, spending most of his time flexing ... while Chrissy Teigen fawns over him from the DJ booth.