spass Spiegel -

The term we associate with the morning, a.m., is an abbreviation the Latin phrase ante merīdiem meaning “before midday”; p.m. is an abbreviation of post merīdiem, meaning—you guessed it—“after midday.” These two terms help keep ambiguity at bay in the 12-hour time system. (Source) Now easy to remember with after midnight a.m. and pasttime of midday p.m.