Green’s Dictionary of Slang

very, absolutely, really, e.g. fair tasty, fair gorgeous.

1880		Henley & Stevenson Deacon Brodie IV tab.VII i: My back’s fair broke.
1887		‘’Arry on the Jubilee’ in Punch 25 June 305/1: By Jingo she’ll see that I’m fair in the know.
1897		W.S. Maugham Liza of Lambeth (1966) 56: Well, it fair frightened my old man.
1903		Bulletin (Sydney) 23 July 17/1: ’E was lying on the rails, and the train cut him fair in two, [...] but they put him together somethink beautiful out in the platform.
1906		Marvel 22 Dec. 636: He’s fair off his chump – mad as a bloomin’ hatter!
1911		L. Stone Jonah 187: An’ fair gave me the creeps thinkin’ I could see the people scratchin’ their way out of the coffin.
1923		‘Bartimeus’ ‘In the Dog-Watches’ in Seaways 19: Tricks! Gorblime, you’d fair bust yourself with laughin’.
1935		(con. 1920s) McArthur & Long No Mean City 147: ‘You’re fair crazy about the fighting,’ he said.
1937		E. Garnett Family from One End Street 129: I’m fair upset.
1943		R. Chandler Lady in the Lake (1952) 84: This town’s fair bulgin’ at the seams.
1954		J. Phelan Tramp at Anchor 92: Some of them inside parties is fair bleedin’ murder.
1965		F.J. Hardy Yarns of Billy Borker 130: They never caught him. Fair sent them up the wall, it did. Be an annoying thing that.
1974		‘P.B. Yuill’ Hazell Plays Solomon (1976) 71: You’ve fair spoiled my day.
1983		J. Byrell (con. 1959) Up the Cross 31: ‘You rotten fart. I orta smack you fair in the gob’.
1998		R.G. Barrett Goodoo Goodoo 40: I’d have riped that microphone out of the dash and shoved it fair up his date.