whom S3 W1 /huːm/ BrE AmE pronoun
[ Language: Old English ; Origin: hwam ]
the object form of ‘who’, used especially in formal speech or writing:
Desperate for money, she called her sister, whom she hadn’t spoken to in 20 years.
She brought with her three friends, none of whom I had ever met before.
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In everyday spoken or written English, people usually use who rather than whom :
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Whom is usually used only in the phrases one of whom , none of whom , some of whom etc.