Source : Webster's Revised Unabridged Dictionary (1913)

Unthinking \Un*think"ing\, a.
   1. Not thinking; not heedful; thoughtless; inconsiderate; as,
      unthinking youth.

   2. Not indicating thought or reflection; thoughtless.

            With earnest eyes, and round unthinking face, He
            first the snuffbox opened, then the case. --Pope.
      -- {Un*think"ing*ly}, adv. -- {Un*think"ing*ness}, n.

Source : WordNet®

unthinking
     adj 1: not exhibiting or characterized by careful thought [syn: {unreflective},
             {unthoughtful}]
     2: without care or thought for others; "the thoughtless saying
        of a great princess on being informed that the people had
        no bread; `Let them eat cake'" [syn: {thoughtless}, {uncaring}]
     3: mentally sluggish [syn: {lumpish}, {lumpen}]

unthinking
     adv : in a thoughtless manner; "he stared thoughtlessly at the
           picture" [syn: {thoughtlessly}, {unthinkingly}] [ant: {thoughtfully}]