Green’s Dictionary of Slang

1. to suffer bravely, irrespective of the pain.

1990		J. Wambaugh Golden Orange (1991) 348: You’re doing it the hard way. Awful time to white-knuckle it.
1997		L. Davies Candy 23: I would [...] leave my dope at Mason’s house, and white-knuckle it for twenty four hours as a test of strength.
2018		J.-P. Jordan Joys of War 104: I didn’t want to push the PTSD issue to the surface. [...] In hindsight, I was only white-knuckling my recovery .

2. (US) to grasp very firmly.

2017

T. Pluck Bad Boy Boogie [ebook] Jay white-knuckled the wheel, heading to Tony’s shop. Fire burning in his gut.