knuckle up

knuckle up (third-person singular simple present knuckles up, present participle knuckling up, simple past and past participle knuckled up)

1. (informal, chiefly African-American Vernacular) To clench one's fist(s) as though in preparation for a fight.
   • 2012, Vanessa Miller, Latter Rain:

     With a resolve to get his son away from these monsters, Isaac knuckled up and mean-mugged his opponent.

   • 2012, Pink Maxwell, A Scorpion's Sting, page 209:

     Tony stumbled a little and when he found his balance, he knuckled up and sent a jab toward Will's neck.

2. (informal, by extension) To approach a problem aggressively.