The phrase is American English slang although it is now being used increasingly on this side of the Atlantic. Terry Pratchett used it as the title for a novel in 2004.
Meaning to become uncontrollably angry, it originates in a series of events in the USA in the 1980s and 1990s. During those two decades well over 40 people were killed in incidents when workers for the United States Postal Service ran amok and shot guns randomly at fellow employees.
In August 1986, for instance, a postman in Oklahoma shot and killed 14 people in the post office where he worked before turning the gun on himself. It’s unclear why US postal workers should be particularly liable to bouts of murderous rage but the phrase ‘going postal’ entered the language in the mid-1990s.
Answered by: Nick Rennison, author and journalist