"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).
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-).
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).