Fly into a violent rage, especially when provoked by workplace stress.
This originated in the USA in 1990s following a several incidents from 1986 onward, in which individuals working for the United States Postal Service workers shot and killed fellow workers and members of the public. The first event of this kind was in August 1986, when fourteen postal workers were killed and six wounded by gunshot in an Oklahoma post office, by Patrick Sherrill, himself a postal worker, who later shot himself. Between that date and 1997 there were more than 40 deaths in 20 such incidents.
The term was first recorded in the Florida newspaper The St. Petersburg Times, December 1993:
"The symposium was sponsored by the U.S. Postal Service, which has seen so many outbursts that in some circles excessive stress is known as 'going postal'."
Despite the unfortunate reputation gained by the US Postal Service as a result of these murders, statistics measured over the longer term show that the service is no more stressful than other comparable occupations and the number of incidents of workplace rage no worse than other forms of employment.
See other phrases that were coined in the USA.