Merciless, a. — Webster enters it under “merciless”
Destitute of mercy; cruel; unsparing; -- said of animate beings, and also, figuratively, of things; as, a merciless tyrant; merciless waves. The foe is merciless, and will not pity. Shak.
Syn. -- Cruel; unmerciful; remorseless; ruthless; pitiless; barbarous; savage. -- Mer"ci*less*ly, adv. -- Mer"ci*less*ness, n.
Shades of meaning
Fernald lists mercilessness only as an antonym, under pity.
He lists it without comment; the entries above explain its neighbors.
Synonyms
none given