上午 a.m (am) 的全拼是 ante meridiem； 下午 p.m (pm) 的全拼是 post meridiem。

应该是从拉丁文来的。

• ante meridiem: before noon; "let's meet at 11 A.M."
  wordnet.princeton.edu/perl/webwn

• before noon (used after a time to indicate it is between midnight and noon, i.e., in the morning); ad modum (less commonly used)
  en.wiktionary.org/wiki/a.m.

• post meridiem: between noon and midnight; "let's meet at 8 P.M."
  wordnet.princeton.edu/perl/webwn

• after noon (used after a time to indicate it is between noon and midnight, i.e., in the afternoon, evening or night as opposed to the morning)
  en.wiktionary.org/wiki/p.m.