absent-minded
adjective
ab·sent-mind·ed ˌab-sənt-ˈmīn-dəd 
variants or less commonly absentminded
1
a
: lost in thought and unaware of one's surroundings or actions : preoccupied
was too absent-minded to notice what time it was
b
: tending to forget or fail to notice things : given to absence of mind (see absence sense 3)
an absent-minded person prone to forgetting appointments
2
: indicative of or resulting from preoccupation or absence of mind
gave an absent-minded reply
absentmindedly adverb
or less commonly absent-mindedly
absentmindedness noun
or less commonly absent-mindedness
Synonyms
- absent
- abstracted
- distracted
- preoccupied
See all Synonyms & Antonyms 
Example Sentences
Her absentminded husband forgot their anniversary. She did the chores in an absentminded way.
Recent Examples on the Web But magnet faculty enjoyed a great deal of autonomy—Anstead, who developed a reputation among Core students for being gentle and brilliant, if a bit absentminded, gave teachers free rein over their classes. 
Longreads, 16 Mar. 2012 But as happens to absentminded professors worldwide (at least according to my dear wife), my mind started to wander from the task at hand to something related to plants. Paul Cappiello, The Courier-Journal, 6 Mar. 2020 In their own seemingly innocent, absentminded way, slow drivers pose just as much of a threat to society as fast ones. Joe Queenan, WSJ, 12 Apr. 2018 Megyn Kelly’s dog took a brief sojourn in a total stranger’s lavish Montana villa after her absentminded caretakers unintentionally abandoned her there during their vacation in the Big Sky Country. Amanda Arnold, The Cut, 26 Mar. 2018 Over on the side, two men played an absentminded game of pool, more interested in the discussion than the 6 in the side pocket. The Root, 16 June 2017
Longreads, 16 Mar. 2012 But as happens to absentminded professors worldwide (at least according to my dear wife), my mind started to wander from the task at hand to something related to plants. Paul Cappiello, The Courier-Journal, 6 Mar. 2020 In their own seemingly innocent, absentminded way, slow drivers pose just as much of a threat to society as fast ones. Joe Queenan, WSJ, 12 Apr. 2018 Megyn Kelly’s dog took a brief sojourn in a total stranger’s lavish Montana villa after her absentminded caretakers unintentionally abandoned her there during their vacation in the Big Sky Country. Amanda Arnold, The Cut, 26 Mar. 2018 Over on the side, two men played an absentminded game of pool, more interested in the discussion than the 6 in the side pocket. The Root, 16 June 2017 Word History
First Known Use
1809, in the meaning defined at sense 1a
Kids Definition
absentminded
adjective
ab·sent·mind·ed ˌab-sənt-ˈmīn-dəd
: tending to forget or not pay attention
Two absentminded students forgot their homework today.