Knuckle down
Definition 1 of 2
Pro Tip 1/3Serious Effort
Use 'knuckle down' when a big effort is needed to complete work. 
With finals week approaching, I must knuckle down and revise.
Pro Tip 2/3Time Management
'Knuckle down' often suggests managing time better to focus on tasks. 
Stop procrastinating and knuckle down on your project.
Pro Tip 3/3Mood Shift
Indicates a change from a relaxed to a disciplined state of mind. 
After the break, I knuckled down to write my thesis.