absentminded
1. [ adjective ] lost in thought; showing preoccupation
Synonyms: abstracted scatty absent
Examples:

"an absent stare" "an absentminded professor" "the scatty glancing quality of a hyperactive but unfocused intelligence"

Related terms: inattentive
Similar spelling:   absentmindedly
  abashment
  abasement
  absenteeism