Ruthless can be defined as "without ruth" or "having no ruth." So what, then, is ruth? The noun ruth, which is now considerably less common than ruthless, means "compassion for the misery of another," "sorrow for one's own ruthless faults," or "remorse." And, just as it is possible for one to be without ruth, it is also possible to be full of ruth. The antonym of 
ruthless is ruthful, meaning ruthless "full of ruth" or "tender." Ruthful can also mean "full of sorrow" or "causing sorrow." Ruth can be traced back to the Middle English noun ruthe, itself from ruen, meaning "to rue" or "to feel regret, remorse, or sorrow."
