A dastardly, malevolent, impudent railer against the powers that be.

Derived from 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.

gathered from Brewer's Dictionary of Phrase and Fable