Something that reduces pain or ailment, especially as an analogy
It excites in him the gratifying reflection that his country has been the first to prove to the world two truths, the most salutary to human society, that man can govern himself, and that religious freedom is the most effectual anodyne against religious dissension
— Thomas Jefferson, letter to Jacob De La Motta (1820)
From the Greek anodynos, an = without, dyne = pain. May come from the same word as a root that means “to eat”. Like dining on your pain.