1. begin to work hard at something
knuckle down to work/study
'Knuckle down' means to take a task seriously and to work hard at it. This idiom is usually applied to studying and work. "I need to knuckle down if I want to be prepared for my exam on Friday." "I haven't been very productive at work recently but my boss has made it clear that I need to knuckle down." Similar in meaning to ' buckle down' ' pull your socks up' and 'put your head down'.