ab·sent·mind·ed
 /ˌæbsəntˈmaındəd/  adj  
  : tending to forget things or to not notice things : having or showing a lack of attention
   Her absentminded husband forgot their anniversary.
   She did the chores in an absentminded way.
  ab·sent·mind·ed·ly  adv  
   Her husband absentmindedly forgot their anniversary.
  ab·sent·mind·ed·ness  noun   [  noncount  ]
   She mislaid her keys in a fit of absentmindedness. [=absence of mind]