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Adjective: absentminded  ,ab-sunt'mIn-did
  1. Lost in thought; showing preoccupation
    "an absentminded professor";
    - absent, abstracted, scatty [Brit, informal]
     
  2. Tending to forget or fail to notice things due to preoccupation with one's thoughts
    "He was so absentminded he put his keys in the refrigerator"

See also: inattentive, unattentive [rare]

Encyclopedia: Absentminded