fem. proper name, biblical ancestor of David, from Hebrew Ruth, probably a contraction of reuth "companion, friend, fellow woman." The Old Testament book tells her story.
c. 1200, perhaps late Old English, ruthe, "misery, sorrow, grief;" also "pity, compassion, sorrow for the misery of another" (often in have ruth, take ruth); also "remorse, repentance, regret;" 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).