Thoughtless, adv. — Webster enters it under “thoughtless”
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.
Shades of meaning
Fernald has no entry of its own for thoughtlessness; he lists it under neglect.
He lists it without comment; the entries above explain its neighbors.