Does this word seem pugnacious? Well, if your prognosis for pugnacity involves lots of jutting jaws, you just might have it. But you gneed gnot gnash your teeth… prognathous really describes a kind of facial physiognomy. And, yes, the g is pronounced as written, which means your tongue does a one-two back-front touch that might make you think of a stuffed-up nose but won’t necessarily make you jut your jaw much.
And it’s the jutting jaw that is the essence here: Greek pro “forward” and gnathos “jaw”. This word always makes me think of Philip IV of Spain – take a little time to look at any of the several portraits of him at various ages all made by Diego Velázquez. But others might sooner think of, say, Jay Leno. For the clinical sense, however, the jaw need not be as long as all that; it just needs to jut at a sharper-than-average angle.
There is something about this word that feels right to me for a big jaw. The gn seems to have the right feel, but it’s not just that. While magnum may seem jaw-y, cognition doesn’t really. The visual effect, even when the g is not said, may be a bit of a cue, but while gnash has a similar mandibularity, gnat has not. So it’s hard to separate out true phonaesthetic effect from simple awareness of the sense filtering through.
Still and all, tell me how, for instance, the name Gnaeus Naevius strikes you. Does it seem rather growly and toothy or otherwise pugnacious, perhaps like Gnasher, the nasty dog of that nasty British boy Dennis the Menace (no, not the American one drawn by Hank Ketcham; this one, started on the other side of the pond and appearing first a mere three days after the American one hit the presses in 1951, is unrelated). Gnasher has been know to chuckle gneh-heh, so why might he not, when gnawing on a bone, growl Gnaeus Naevius? Well, in fact, Gnaeus Naevius was a Roman satirical playwright, known for making the patricians gnash their teeth. There is no evidence that he was prognathous. But doesn’t his name have that feel?
On the other hand, there is also no evidence that he was opisthognathous. There is nothing at all to tell us about his gnathic index, in fact.
OK, what and what? Gnathic index: an indicator of how relatively far forward the jaw does or doesn’t jut. Opisthognathous: why, the opposite of prognathous, of course, and a word just made for lisping weak-jawed sorts to say.