Knuckle down or

Knuckle down to something

It means to start working diligently and apply oneself to the work on hand seriously.

Anothe expression is to Buckle down. It means the same thing. However, it isn’t used as commonly.

Some sentences that show the practical use of this idiom are:

If you’ve procrastinated on your project, it’s time now to knuckle down and finish it.

If you really want to get a scholarship, you’ll have to knuckle down and start studying seriously.

Lunch is over. Knuckle down fellas!

The exams are next week. I’m going to have to knuckle down to some serious study.

We’ve got this project after a tough fight. And well have to finish it in time. So knuckle down to it guys!