Absentminded (adjective)

1. Distracted and unaware of one's surroundings or current situation.
2. Forgetful and easily distracted.

Origin:

From absent + minded, from the french absenté ("absent") + -ment ("-men.t")

Examples:

1. He was so absentminded that he left his keys in the door.
2. She was absentmindedly staring out the window during the meeting.
3. His absentmindedness resulted in him forgetting his lunch at home.
4. She became absentminded after her mother's death.
5. His absentmindedness often caused him to miss important deadlines.