"pertaining to the mail system," 1843, on model of Fr. postale (1836), from post (3). Noun meaning "state of irrational and violent anger" (usually in phrase going postal) attested by 1997, in ref. to a cluster of news-making workplace shootings in U.S. by what were commonly described as "disgruntled postal workers" (the cliche itself, though not the phrase, goes back to at least 1994).