lose one's mind

Also, lose one's reason. Go crazy, lose one's sanity, as in I thought she'd lost her mind when she said she was going ice-fishing, or That assignment is enough to make me lose my reason. The first expression dates from the late 1500s; the second employs reason in the sense of “unimpaired mental faculties,” a usage dating from the late 1300s. Also see under go out of one's mind; have all one's buttons.