I love the works of Jane Austen, and I completely accept (and mostly enjoy) the fact that language changes constantly (especially now I'm old enough to have seen so many hideous words and expressions slip into merciful obscurity): but, really, Sir Walter Elliiot's description in Austen's Persuasion of his kinsman Mr Walter Elliot took me aback a bit:
He did justice to his very gentlemanlike appearance, his air of elegance and fashion, his good-shaped face, his sensible eye; but, at the same time, "must lament his being very much under-hung...".
Apparently it means that Mr Elliot's had a projecting lower jaw.
Gosh, but it's a word to be careful with, isn't it.
Word Not To Use Today: underhung. This word tends not to have anything to do with jaws, nowadays. It may describe meat that's tough because it's been cooked too soon after slaughter, or a door that slides on runners.
Or may not.
The word hang comes from the Old English hangian, which meant the same thing.