post meridiem
/ˌpəʊs(t) mɪˈrɪdɪəm/adverb
After midday; applied to the hours between noon and midnight. Usually abbreviated P.M., p.m., or pm.
Origin
Mid 17th century; earliest use found in William Lilly (1602–1681), astrologer. From classical Latin post merīdiem after midday from post after + merīdiem, accusative of merīdiēs midday.