What is "knuckled down"

Usage examples of "knuckled down".

But then I dropped a couple of extracurriculars, knuckled down, and brought that grade right up.

I hadn't the wit to perceive it, until the sticking point was reached, and she knuckled down in a flood of tears.

Momentarily startled, Rafferty knuckled down over the steering wheel and glared out at the road unfolding ahead.

Each person visibly knuckled down and braced for whatever impact it would take to shake these hawks from the empress’.

Nevertheless, the CIGA protests made it doubly difficult to organize a workable system of home defense, even though devoted Imperials everywhere knuckled down and worked 'round the clock with whatever resources they could scrape together.