from The American Heritage® Dictionary of the English Language, 4th Edition
- n. Tolerance or immunity to a poison acquired by taking gradually larger doses of it.
from Wiktionary, Creative Commons Attribution/Share-Alike License
- n. The development of immunity to a poison by gradual ingestion of successively greater amounts.
from The Century Dictionary and Cyclopedia
- n. Immunity against the action of a poison acquired by taking the drug in constantly increasing doses: a method said to have been conceived by Mithridates to protect himself against palace intrigues.
After Mithridates VI, who is said to have acquired tolerance for poison.(American Heritage® Dictionary of the English Language, Fourth Edition)
From Mithridates (referring to Mithridates VI of Pontus) and -ism. First attested in 1851. (Wiktionary)