Also called a "lantern jaw" it describes individuals with a protruding lower jaw, or very distinct jaw line, coupled with a usual absence of any chin dimple, or cleft chin. Usually indicative of someone with recent Irish ancestry. Political Analysts Lawrence O'Donnell and Chris Matthews have very pronounced Irish Jaws, as do the dead Kennedy brothers.
"How do you know he is Irish?"
"Look at that Irish Jaw, it's a minute early wherever he goes."