The term originally appeared within the Cybergamer Xbox community, however has spread into the vocabulary of many online gamers.
someone who cannot handle getting hit in a fight very well is said to have a weak chin or a glass jaw. someone who can handle lots of abuse is said to have a strong chin.
dude 2: yeah, but he's still fighting, look at him, it's like the punches aren't even affecting him.
dude 1: he's got a hell of a chin on him, that's for sure.