knuckle down phrasal verb informal (see also knuckle)
to suddenly start working or studying hard Synonym : get down to:
If he doesn’t knuckle down soon, he’ll never get through those exams.
knuckle down to
He is clearly ready to knuckle down to the task.

[TahlilGaran] Dictionary of Contemporary English ▲

knuckle down
to work hard.
You're going to have to knuckle down to improve your grades if you want to get into a good college.

Volunteers really knuckled down and cleaned up the town after the storm.

[TahlilGaran] English Idioms Dictionary ▲

knuckle down
begin to work earnestly
I think it is time that we knuckle down and finish this project.

[TahlilGaran] English Idioms Dictionary ▲

knuckle down
See: BUCKLE DOWN.
{v.} To give complete attention (to an effort or job); attend.
They chatted idly for a few moments then each buckled down to work.

Jim was fooling instead of studying; so his father told him to buckle down.

[TahlilGaran] English Idioms Dictionary ▲