Green’s Dictionary of Slang

an intimate friend.

1857		Paul Pry (London) 15 Aug. n.p.: Have you seen the gray biffin lately ? Don’t forget the lark about the tea-gardens, and who upset the quart of milk!
1883		J. Greenwood Tag, Rag & Co. 239: Ain’t that up to Dick, my biffin?
1885		‘’Arry on Song and Sentiment’ in Punch 14 Nov. 229/1: That’s Life, and no bow-wow, my biffin!
1886		‘’Arry on Commercial Education’ in Punch 26 Sept. in P. Marks (2006) 124: University, charlie, for bagmen [...] / All my eye and a bandbox, my biffin.