Webster’s Revised Unabridged Dictionary · 1913
Thoughtless
Thought′less, adv. 1. Lacking thought; careless; inconsiderate; rash; as, a thoughtless person, or act.
2. Giddy; gay; dissipated. [R.] Johnson.
3. Deficient in reasoning power; stupid; dull.
Thoughtless as monarch oaks that shade the plain. Dryden.
— Thought′less·ly, adv. — Thought′less·ness, n.