knuckle down

Example: “It's time to knuckle down and finish the thesis.”

Origin: Comes directly from the traditional game of marbles, where many regional rule sets require a shooter to keep at least one knuckle touching the ground while taking a shot — quite literally "knuckling down" into the correct shooting position. The phrase in this specific game-related sense is documented as early as 1740, in Thomas Dyche and William Pardon's dictionary of common English words, and by 1864 Webster's Dictionary was already defining it in something close to its now-familiar figurative sense. The transfer from marbles to general effort makes intuitive sense: the steady focus and physical discipline needed to hold a proper shooting stance became, by extension, the same kind of serious, sustained application needed to work hard at any task.