Etymology
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post meridiem 

"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).

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ante meridiem 
"of morning, before mid-day," 1560s, Latin, literally "before noon," from ante "before" (from PIE root *ant- "front, forehead," with derivatives meaning "in front of, before") + accusative of meridies "midday, noon" (see meridian). Adjective antemeridian is attested from 1650s.
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