'em

wiktionary

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From earlier hem, from Middle English hem, from Old English heom(“them”, dative) of hie, [1] originally a dative plural form but in Middle English coming to serve as an accusative plural as well. Cognate with Dutch hun(“them”), German ihnen(“them”).

Now often treated as a form of them, which however derives from Old Norse rather than Old English.

etymonline

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'em (pron.)

Middle English; since 17c. taken as a colloquial abbreviation of them, but originally at least in part a form of hem, dative and accusative of the third person plural pronoun.