E. Cobham Brewer 18101897. Dictionary of Phrase and Fable. 1898.
Said to be derived from the Greek antimonachos (bad for monks). The tale is that Valentine once gave some of this mineral to his convent pigs, who thrived upon it, and became very fat. He next tried it on the monks, who died from its effects; so Valentine said, tho good for pigs, it was bad for monks. This fable is given.by Furetière.
Another derivation is anti-monos (averse to being alone), because it is found in combination with sulphur, silver, or some other substance.
Littré suggests isthimmit, and connects it with stibium.