To knuckle down is to start to work or study very hard.
Examples of use:
a) You need to knuckle down and finish your homework.
b) I haven’t done much revision for my exams – I must knuckle down before it’s too late.
c) I’m not very impressed with your work: knuckle down, or you will have to leave the company.
d) We all knuckled down and finished the work before the deadline.
e) Your employees should spend less time chatting, and knuckle down and get on with their work!
||knuckle down and knuckles down|
Image © Alex Proimos