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"pertaining to the mail system," 1843, on model of French postale (1836), from post (n.3). Noun meaning "state of irrational and violent anger" (usually in phrase going postal) attested by 1997, in reference 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 at least to 1994).
[1990s+ Computer; a grim reference to ''the unfortunate number of postal employees in recent years who have snapped or gone on shooting rampages''—Macon Telegraph]
To pay; fork over: He had ponied up a silver quarter
[1824+; fr earlier British post the pony, ''pay,'' fr 16th-century legem pone, ''money,'' fr the title of the Psalm for Quarter Day, March 25, the first payday of the year]