Is It because they have never been knocked down? Or Is It because they get knocked down but always get back up?
Iron / granite chin for me is where the fighter simply seems to absorb headshots without much in the way of tells / being stunned or otherwise fazed, and for me the key thing here is it's not just taking one or two decent shots, it's taking them throughout the fight and not flinching. Recently Joe Joyce against Parker is what i'd call a textbook definition of a great beard - Parker scored flush bombs repeatedly, but Joyce didn't even seem to blink or pause on any of them.
Taking repeated head shots without visible harm as gollumslave said. So yeah, Joyce would be a perfect candidate here. McCall, Tua and comeback Foreman would be good shouts as well. Maybe Wach before his chin cracked. Getting knocked down and getting back up again is sign of great heart and recuperative abilities, not chin.
Joyce, Foreman, Golovkin, McCall (Tyson's sparring partner, kind of a pre-requisite) are in a tier of their own Being knocked down by a sledgehammer but being able to get back up (Holmes getting up vs Shavers, Fury getting up from Wilder) is heart + recovery, not neccessarily chin. Fury has gone down like 6/7 times in his career, over 34 fights, that's not a great ratio for him.
Iron chins are hit and tested ( hit by ) by punchers ( a few men ) several times...and they pass with being knocked out by punches. Rare in boxing. That is my definition.
I agree its iron, but he was close to going down and hurt vs Bruce Seldon in round 9. He was close to going down. Watch it. Vitali Klitschko was never down as well and tested and hit by known punchers.
I sat in front of Nigel Benn recently and he said its all in the neck! He said hitting Eubank was like hitting a wall because of his strong neck.
Who was the best natural middle puncher Hagler fought? I agree Tommy Hearns was a great puncher but he only last 3 rounds and was done for in two. Hagler has the qualifications for an iron chin.