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