absent-minded
Definitions
Having or showing a forgetful or inattentive disposition.
Lost in thought and unaware of one's immediate surroundings.
Examples
Being rather absent-minded, he often forgot where he had placed his car keys.
Her absent-minded professor was so lost in thought that he walked into the classroom wearing two different shoes.
Synonyms
forgetful distracted preoccupied scatterbrained dreamy inattentive oblivious