cumulative genitive
Definition - A noun phrase that shows possession by using both an s-genitive and an of-genitive.

Example -
He is a friend of my brother's.

Oxford English Dictionary -
The term's first citation is from 1892:
"The Cumulative or Double Genitive, a peculiarly English combination, where both the of and the s are retained, as ‘that boy of Norcott's’."
(Earle Philol. Eng. Tongue (ed. 5) 547)

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