dewlap
The pendulous skin under the neck of an ox, or a similar feature on any other animal.
Fire burst from its open mouth, its eyes glowed with a smouldering glare, its muzzle and hackles and dewlap were outlined in flickering flame.
1901 – 1902, Arthur Conan Doyle, The Hound of the BaskervillesThe sagging flesh on the throat of an elderly human.
He ended up looking at himself in the mirror. His image stared back at him. He was developing a dewlap - a definite dewlap. He waggled under his chin scornfully with his fingers.
Where it comes from
From Middle English dewlappe. The first element may be dew (and if not, seems to have been altered by folk etymology to resemble it); the second element is lap (“loose fabric”), Old English læppa (“a loose hanging piece”); compare jellop and jowlop (from jowl + lap), and dewclaw. Compare old Norwegian and Danish doglæp (the modern Danish word for "dew" is dug, but compare Old Norse dǫgg (“dew”) and Norwegian dogg). Old English instead terms such skin the frǣtlæppa (whence Middle English fresh-lappe).
Adapted from the Wiktionary entry dewlap (CC BY-SA 4.0).