Green’s Dictionary of Slang

(UK juv.) a call for a truce during a game, or a statement that one is ineligible for a given duty or command.

1824		Pierce Egan’s Life in London 19 Sept. 269/1: ‘Faiks then [...] I’ll lay you, you dirty blackguard, that you do not lick me’’.
1860		C.J. Lever One of Them I 235: He was suddenly aroused by the telegraph clerk’s demand for thirty francs. ‘Thirty francs for four words?’ ‘You might send twenty for the same sum,’ was the bland reply. ‘Faix, and so we will,’ said Joe.
1870		N&Q Ser. 4 VI 415/2: ‘Fains’, or ‘Fain it’ – A term demanding a ‘truce’ during the progress of any game, which is always granted by the opposing party.
1889–90		Barrère & Leland Dict. of Sl., Jargon and Cant.
1891		Sporting Times 4 Apr. 4/4: Steady with that biceps, and fain larx.
1904		Sydney Sportsman (Surry Hills, NSW) 25 May 3/3: ‘Fain dubs – put one back!’.
1912		E. Pugh Harry The Cockney 75: ‘Fainitz!’ he spluttered.
1913		C. Mackenzie Sinister Street I 103: He could shout ‘fain I’ to be rid of an obligation.
1935		S. Kingsley Dead End Act II: milty, also shouting quickly, topping split and holding up crossed fingers: Fens! No akey! No akey!
1948		J. Betjeman ‘Beside the Seaside’ Sel. Poems 82: ‘You ask him, Jennifer.’ ‘No – Michael? – Anne?’ / ‘I’d rather not.’ ‘Fains I.’ ‘It’s up to you.’.
1959		I. & P. Opie Lore and Lang. of Schoolchildren (1977) 160: The truce term ‘fains’, ‘fannies’, or ‘faynights’.
1980		Barltrop & Wolveridge Muvver Tongue 65: ‘Fainits’ [...] is a key word in East End children’s lore, and means ‘truce’.
1989		H. Harrison Bill [...] on the Planet of Robot Slaves (1991) 18: ‘Finns . . . Uncle!’ Praktis gasped.
1999		(con. 1920s–30s) G. O’Neill My East End (2000) 101: And crossed fingers and ‘fainlights’ gave you protection.
2001		OnLine Dict. of Playground Sl. 🌐 faynights, fainites a ‘shout’ (often accompanied by crossed fingers) created temporary immunity from being made ‘it’ when playing sticky toffee, stuck in the mud, tag, etc.