What does "go off" mean?

Explode, detonate; also, make noise, sound, especially abruptly. For example, I heard the gun go off, or The sirens went off at noon. This expression developed in the late 1500s and gave rise about 1700 to the related go off half-cocked, now meaning “to act prematurely” but originally referring to the slipping of a gun's hammer so that the gun fires (goes off) unexpectedly.

Leave, depart, especially suddenly, as in Don't go off mad, or They went off without saying goodbye. [c. 1600]

Keep to the expected plan or course of events, succeed, as in The project went off smoothly. [Second half of 1700s]

Deteriorate in quality, as in This milk seems to have gone off. [Late 1600s]

Die. Shakespeare used this sense in Macbeth (5:9): “I would the friends we missed were safely arrived.—Some must go off.”

Experience orgasm. D.H. Lawrence used this slangy sense in Lady Chatterley's Lover (1928): “You couldn't go off at the same time....” This usage is probably rare today. Also see get off, def. 8.

go off on a tangent. See under on a tangent.

go off one's head. See off one's head. Also see subsequent idioms beginning with go off.