Thoughtless, 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.
Shades of meaning
Fernald has no entry of its own for thoughtless; he lists it under abstracted.
Where he writes about it:
A preoccupied person may seem listless and thoughtless, but the really listless and thoughtless have not mental energy to be preoccupied. — from abstracted