scatterbrain

/ˈskætərbreɪn/
noun
  1. A person who is forgetful, disorganized, or unable to focus their thoughts.
    • The scatterbrain in the office always misplaces important documents.
    • My brother is such a scatterbrain; he lost his keys three times today.
    • Don't be a scatterbrain — write down your appointments so you don't forget them.
Synonyms
Antonyms