Shirt of Nessus. A source of misfortune from which there is no escape; a fatal present; anything that wounds the susceptibilities. Thus Renan has “the Nessus-shirt of ridicule.” Hercules ordered Nessus (the centaur) to carry his wife Dejanira across a river. The centaur ill-treated the woman, and Hercules shot him with a poisoned arrow. Nessus, in revenge, gave Dejanira his tunic, saying to whomsoever she gave it would love her exclusively. Dejanira gave it to her husband, who was devoured by poison as soon as he put it on; but, after enduring agony, the hero threw himself on a funeral pile, and was consumed. (See Harmonias Robe.)
While to my limbs th'en venomed mantle clings, Drenched in the centaur's black, malignant gore.
West: Triumphs of the Gout (Lucian).
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894