absentminded

adjective

1. so lost in thought that one does not realize what one is doing, what is happening, etc.; preoccupied to the extent of being unaware of one’s immediate surroundings.

adjective

1. preoccupied; forgetful; inattentive

adj.

also absent-minded, “preoccupied,” 1810, from absent + minded. Related: Absentmindedly; absentmindedness.