From Longman Dictionary of Contemporary Englishsomebody’s mind is wanderingsomebody’s mind is wanderingMENTALLY ILLused to say that someone has become unable to think clearly, especially because they are old → wander
Examples from the Corpus
somebody’s mind is wandering• My mind is wandering, as Isabel has said, I forget things.• Paul finds his mind is wandering, thinking about the poll tax program again. 