Refers to the finger joints, especially in contexts like martial arts, anatomy, or casual speech.
He punched his opponent with his knuckles.
knuckleball
Baseball term for a pitch with little spin; often used in the full form ナックルボール.
See also: ナックルボール
His knuckleball is hard to hit.
He has a habit of cracking his knuckles when he's nervous.
That pitcher throws a knuckleball as his signature pitch.
He ended up punching his opponent with his knuckles.