Here are some expressions using the word KNUCKLE:
1. Knuckle down (verb)
Meaning: to make a great effort
• I’ve got my exams next week and I haven’t done much work, so I’d better knuckle down.
2. Knuckle under (verb)
Meaning: to submit to authority
• The teacher made the students knuckle under and hand their projects in on time.
3. Rap on the knuckles (idiom)
Meaning: to be punished or reprimanded, not too severely, but to remind not to do that again.
• Ryan got a rap on the knuckles for leaving the kitchen in a mess.
4. Knucklehead (noun)
Meaning: a stupid person
• I’d like to get my hands on the knucklehead who designed that paper-towel dispenser.
5. Knuckle sandwich (idiom)
Meaning: a punch in the mouth
• If he keeps bugging me, he surely will get my knuckle sandwich!