"after noon, occurring after the sun has passed the meridian," applied to the time between noon and midnight, 1640s, Latin, from post "after" (see post-) + accusative of meridies "midday, noon" (see meridian).
Entries linking to post meridiem
post-
word-forming element meaning "after," from Latin post "behind, after, afterward," from *pos-ti (source also of Arcadian pos, Doric poti "toward, to, near, close by;" Old Church Slavonic po "behind, after," pozdu "late;" Lithuanian pas "at, by"), from PIE *apo- (source also of Greek apo "from," Latin ab "away from" see apo-).
meridian n.
mid-14c., "noon, midday," from Old French meridien "of the noon time, midday; the meridian; a southerner" (12c.), and directly from Latin meridianus "of midday, of noon, southerly, to the south," from meridies "noon, south," from meridie "at noon," altered by dissimilation from pre-Latin *medi die, locative of medius "mid-" (from PIE root *medhyo- "middle") + dies "day" (from PIE root *dyeu- "to shine").
The cartographic sense of "a great circle or half-circle of a sphere passing through the poles" is attested from late 14c., originally astronomical. Figurative uses tend to suggest "point of highest development or fullest power," implying a subsequent decline. As an adjective from late 14c. Related: Meridional. The city in Mississippi, U.S., was settled 1854 (as Sowashee Station) at a railway junction and given its current name in 1860, supposedly by people who thought meridian meant "junction" (they perhaps confused the word with median).
p.m.
abbreviation of Latin post meridiem "after noon," attested from 1660s.