Mithridates, he died old. Housman's passage is based on the belief of the ancients that Mithridates the Great [c. 135-63 B.C.] had so saturated his body with poisons that none could injure him. When captured by the Romans he tried in vain to poison himself, then ordered a Gallic mercenary to kill him.
Source: A Shropshire Lad, 1896, no. 62 , st. 4
Contributed by: Zaady