To knuckle down is to start working seriously at last. Time to knuckle down, knuckle down to it, knuckled down for the exams.
It comes from marbles, where a player put a knuckle to the ground to shoot properly. It is different from knuckling under, which means submitting rather than applying yourself.
1phraseknuckle downTo start working seriously at last.
“Your exams are in a month, so it's time to knuckle down.”
knuckle down is a applying.
From the game of marbles, where a player set a knuckle to the ground to shoot; recorded in American English from 1864.
1864 in American English, from the game of marbles.
Etymology from Wiktionary, CC BY-SA
Practised in Phrasal Verbs