Lose one's mental stability, become insane, as in
After he heard that the ore he had mined was worthless, the prospector went out of his mind . This phrase is often used as a humorous exaggeration, as in
I'm going out of my mind reading these proofs. The phrase
out of one's mind dates from the late 1300s, and was used with
go shortly thereafter. Also see
lose one's mind.