knuckle down

To knuckle down means to work very hard doing something and getting it accomplished. This phrase is often used in reference to someone who has a large project or job to get completed either at work or in school and in order to get all of the work done in a short amount of time; they must knuckle down and do it. The word knuckle is used in a variety of idioms as a way to describe trying something with a large amount of effort.