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fain (fen, to forbid) - Chiefly [Brit] School slang, orig. dial.
Used in the expression fains or fain(s) I, fain it, fainit(e)s: see quots. [EA]

1870 N. & Q. 4th Ser. VI. 415/2 ‘Fains’, or ‘Fain it’{em}A term demanding a ‘truce’ during the progress of any game, which is always granted by the opposing party. Ibid. 517/1 A boy who had ‘killed’ another at marbles, that is hit his marble, would call out ‘Fain it’, meaning ‘You mustn't shoot at me in return’; or if a boy was going to shoot, and some inequality of surface was in his way, which he would have cleared away, his antagonist would prevent him by calling out ‘Fain clears’. Ibid. 517/2 If a prefect wants anything fetched for him and does not say by whom, those who wish to get off going say ‘Fain I’. 1889 BARRÈRE & LELAND Dict. Slang, Faints [sic], in vogue among schoolboys to express a wish temporarily to withdraw from participation in the particular sport or game being played. 1891 FARMER Slang, Fains! Fainits! Fain it! 1913 C. MACKENZIE Sinister St. I. I. vii. 103 He could shout ‘fain I’ to be rid of an obligation and ‘bags I’ to secure an advantage. 1927 W. E. COLLINSON Contemp. English 14 The custom of putting oneself out of the game altogether by crossing the fingers and saying pax! or faynights! [feinaits] or both together. 1948 J. BETJEMAN Coll. Poems (1958) 150 ‘I'd rather not.’ ‘Fains I.’ ‘It's up to you.’ 1960 Guardian 1 July 9/7 The Englishman..could remain absolutely pax and fainites. 1969 I. & P. OPIE Children's Games i. 18 This rule is so embedded in children's minds that their immediate response to the proposal of a game is to cry out..‘Me fains first’. Ibid., He must safeguard himself by saying in one gulp, ‘Let's-play-Tig-fains-I-be-on-it’. [OED online]