I was in a meeting recently where we talked about the need to be "ruthless" in determing what would be allowed to be included in a specific event we were planning.
Someone asked, "What does 'ruthless' mean? If I were not ruthless, would I be 'full of ruth'?"
Her question got me to wondering. And here's the answer:
c.1327, from reuthe "pity, compassion" (c.1175), formed from reuwen "to rue" (see rue (v.)) on the model of true/truth, etc. Ruthful (c.1225) has fallen from use since late 17c. except as a deliberate archaism.
So, there you have it. I trust you will be ruthful today when appropriate.