Derived form Thersites, a deformed, scurrilous officer in the Greek army which went to the siege of Troy. He was always railing at the chiefs, and one day Achilles felled him to the earth with his fist and killed him. (Homer: Iliad.)

"He squinted, halted, gibbous was behind,
And pinched before, and on his tapering head
Grew patches only of the flimsiest down.
... Him Greece had sent to Troy,
The miscreant, who shamed his country most."
Cowper's Translation, book ii.

Thesites is a character in Shakespeare's Troilus and Cressida. A malformed Greek, Thersites wanders around the Greek camp commentating cynically on all around him. His protector, Achilles is the only person who doesn't despise him (rather ironic given what Homer says about his death)

Ina similar way to the Fool in King Lear, Thersites often provides the most insight into events. He speaks the famous lines

. . . lechery, lechery; still,
wars and lechery; nothing else holds fashion."