A.M. also is an acronym for Anno Mundi, or in the year of the world, which was previous to AD.

It is also an acroym for Ante Meridiem, or before noon

Or, Amplitude Modulation as is 790 AM KNST Tucson, Arizona.

Or, Airmail.

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Notes:  1  : A
        b3 : C
        5  : E

Am (#). [AS. am, eom, akin to Gothic im, Icel. em, Olr. am, Lith. esmi, L. sum., Gr. , Zend ahmi, Skr. asmi, fr. a root as to be. . See Are, and cf. Be, Was.]

The first person singular of the verb be, in the indicative mode, present tense. See Be.

God said unto Moses, I am that am. Exod. iii. 14.

© Webster 1913.