Merciless, a.
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 has no entry of its own for merciless; he lists it under barbarous.
He lists it without comment; the entries above explain its neighbors.