1
general::
Phrase(s): knuckle down (to something)
Fig. to get busy doing something. • I want you to knuckle down to your work and stop worrying about the past. • Come on. Knuckle down. Get busy.
McGrawhill's American Idioms And Phrasal Verbs
2
general::
1. Apply oneself seriously to some task or goal, as in The professor insisted that we knuckle down and get our papers in by Friday. Both this term and the rhyming synonym buckle down date from the 1860s, but the precise allusion in either is unclear. 2. See KNUCKLE UNDER.
American Heritage Idioms