Green’s Dictionary of Slang

(orig. US) terrifying, very frightening, often of a fairground ride or horror film.

1976		Business Week 26 July 119/2: A less extreme, cheaper, and yet often effective course for the ‘white knuckle’ passenger is to join a fairly new type of therapy group devoted to taking the fear out of flying .
[1979		K. Bonfiglioli After You with the Pistol (1991) 364: My knuckles were now Whiter-Than-White].
1993		K. Lette Foetal Attraction (1994) 22: If she hadn’t been so lost in the Tunnel of Love, this would have been the first clue to the emotional white-knuckle ride that was about to begin.
2000		Guardian Guide 29 Jan.–4 Feb. 13: Staring glumly at an incipient bald spot is hardly the thrill-a-minute white-knuckle shuffle today’s big-screen audiences demand.