am and pm are latin words. am means ante meridiem where ante is before so am is before noon. pm is post meridiem where post mean after. So pm is after noon : see related link for additional...
AM = "Ante - Meridian" PM = "Post - Meridian" "Ante" and "post" mean "before" and "after". "Meridian" is the imaginary line in the sky that runs north/south and passes directly over you.