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Douglas Harper's Etymology Dictionary
ante meridiem

1560s, Latin, literally "before noon," from ante (see ante) + accusative of meridies "midday, noon" (see meridian).

Wiktionary
ante meridiem

adv. before noon, in the morning.

WordNet
ante meridiem
  1. adj. before noon [syn: a.m.]

  2. adv. before noon; "let's meet at 11 A.M." [syn: A.M.]