|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)