The Collaborative International Dictionary of English v.0.48:

Absent-minded \Ab`sent-mind"ed\, a.
   Absent in mind; abstracted; preoccupied. --
   Ab`sent-mind"ed*ness, n. -- Ab`sent-mind"ed*ly, adv.
   [1913 Webster]

WordNet (r) 3.0 (2006):

absentminded
    adj 1: lost in thought; showing preoccupation; "an absent
           stare"; "an absentminded professor"; "the scatty glancing
           quality of a hyperactive but unfocused intelligence"
           [syn: absent, absentminded, abstracted, scatty]

Moby Thesaurus II by Grady Ward, 1.0:

63 Moby Thesaurus words for "absent-minded":
   Lethean, absent, absorbed, abstracted, amnestic, bemused, blocked,
   castle-building, converted, daydreaming, daydreamy, distracted,
   distrait, dreaming, dreamy, drowsing, ecstatic, elsewhere,
   engrossed, faraway, forgetful, forgetting, half-awake, heedless,
   in a reverie, in a trance, in the clouds, inadvertent, inattentive,
   inclined to forget, lost, lost in thought, meditative, mooning,
   moonraking, museful, musing, napping, nodding, oblivious, off,
   pensive, pipe-dreaming, preoccupied, rapt, repressed,
   somewhere else, stargazing, suppressed, taken up, transported,
   unaware, unconscious, unheedful, unheeding, unmindful, unnoticing,
   unobserving, unperceiving, unseeing, withdrawn, woolgathering,
   wrapped in thought