An object form of 'who,' used after prepositions or as the object of a verb in very formal English. In everyday speech, people often just use 'who' instead.
From Old English 'hwām' or 'hwǣm', the dative (object) form of 'hwā' ('who'). It survived mainly in formal writing as English lost many of its case endings.
English used to mark grammar with different word endings, like many other languages still do. 'Whom' is a leftover piece of that older system, which is why it feels so formal now.
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