Green’s Dictionary of Slang

white-knuckle v.

1. to suffer bravely, irrespective of the pain.

[US]J. Wambaugh Golden Orange (1991) 348: You’re doing it the hard way. Awful time to white-knuckle it.
[Aus]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.
[Ire]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 .
D. Soh End of Gender 274: Those who find themselves white-knuckling it, struggling to keep their opinions to themselves [etc].

2. (US) to grasp very firmly.

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