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Douglas Harper's Etymology Dictionary
post meridiem

"after noon," 1640s, Latin, from post "after" (see post-) + accusative of meridies (see meridian).

Wiktionary
post meridiem

adv. after noon, in the afternoon.

WordNet
post meridiem
  1. adj. after noon [syn: p.m., pm]

  2. adv. between noon and midnight; "let's meet at 8 P.M." [syn: P.M., PM]