" absentminded "

1: lost in thought and unaware of one's surroundings or actions;

  * e.g., " ... the grieving woman was so absentminded that she left her key in the lock after opening the door "

•  Synonyms : 

•  absent,

•  abstracted,

•  distracted,

•  preoccupied

•  Antonyms : 

•  alert

2: inclined to forget what one has learned or to do what one should;

  * e.g., " ... an absentminded aunt who some years sends me two birthday checks "

•  Synonyms : 

•  forgetful

•  Antonyms : 

•  retentive