Green’s Dictionary of Slang

a general intensifier; a euph. for fucking adj.

1959		K. Waterhouse Billy Liar (1962) 46: Well which one of them’s got the naffing engagement ring?
1976		Clement & La Frenais Another Stretch of Porridge 16: Stealing your tin of naffing pineapple chunks? Not even my favourite fruit [OED].
1986		F. Taylor Auf Wiedersehen Pet Two 277: I mean, someone’s made him my naffing chaperone.
1992		L. Cody Dupe 20: That naffing Northern Line proper drains you. Bleeding cattle-cars.