"sorrow for the misery of another; repentance, regret," c.1200, ruthe, from Old Norse hryggð "ruth, sorrow," from hryggr "sorrowful, grieved" (see rue, v.) + Proto-Germanic abstract noun suffix *-itho (see -th(2)). Or else formed in English from reuwen "to rue" on the model of true/truth, etc. The Old English word was rue, n.2.