The Collaborative International Dictionary of English v.0.48:

Thoughtless \Thought"less\, adv.
   1. Lacking thought; careless; inconsiderate; rash; as, a
      thoughtless person, or act.
      [1913 Webster]

   2. Giddy; gay; dissipated. [R.] --Johnson.
      [1913 Webster]

   3. Deficient in reasoning power; stupid; dull.
      [1913 Webster]

            Thoughtless as monarch oaks that shade the plain.
                                                  --Dryden.
      [1913 Webster] -- Thought"less*ly, adv. --
      Thought"less*ness, n.
      [1913 Webster]

WordNet (r) 3.0 (2006):

thoughtlessly
    adv 1: in a thoughtless manner; "he stared thoughtlessly at the
           picture" [syn: thoughtlessly, unthinkingly,
           unthinking] [ant: thoughtfully]
    2: showing thoughtlessness; "he treated his parents
       thoughtlessly" [ant: thoughtfully]