What is the meaning of A.m.?

In the 12-hour period from midnight to noon (the first half of the day) when using the 12-hour clock.

Initialism of ad modum.

Abbreviation of amica manu.: (delivered) by the hand of a friend (put on an address)

Abbreviation of ante meridiem.: a.m., in the forenoon

a.m. (before noon)/(antes do meio dia) (Portuguese language)

Source: wiktionary.org