Senseless, a. — Webster enters it under “senseless”
Destitute of, deficient in, or contrary to, sense; without sensibility or feeling; unconscious; stupid; foolish; unwise; unreasonable. You blocks, you stones, you worse than senseless things. Shak. The ears are senseless that should give us hearing. Shak. The senseless grave feels not your pious sorrows. Rowe. They were a senseless, stupid race. Swift. They would repent this their senseless perverseness when it would be too late. Clarendon. --- Sense"less*ly, adv. -- Sense"less*ness, n.
Shades of meaning
Fernald has no entry of its own for senselessness; he lists it under idiocy.
He lists it without comment; the entries above explain its neighbors.