Em is how the letter M is pronounced.(noun)
An example of em is how you'd say "M" when reciting the alphabet.
Em is defined as a casual or informal way of saying them.(pronoun)
An example of em is telling someone that you called your friends over; you called em over.
See em in Webster's New World College Dictionary
Origin: taken as corruption of them, but also < ME hem, dat. pl. of 3d pers. pron. used as acc. & later replaced by them
See em in American Heritage Dictionary 4
Origin: From Middle English hem
Origin: , from Old English him, heom
Origin: , dative and accusative pl. of hē, he; see he 1.
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