Ruthless, a. — Webster enters it under “ruthless”
Having no ruth; cruel; pitiless. Their rage the hostile bands restrain, All but the ruthless monarch of the main. Pope. -- Ruth"less*ly, adv. -- Ruth"less*ness, n.
Shades of meaning
Fernald lists ruthlessness only as an antonym, under pity.
He lists it without comment; the entries above explain its neighbors.
Synonyms
none given