Clenched Fist
Knuckles
noun, する verb
A fist that's like a bone (hard) is a fist that's clenched and ready to punch, making this a clenched fist or knuckles, if you want to get even bonier about it.
げんこつ
This is a jukugo word that uses the on'yomi readings of the kanji. But 拳 doesn't use the one you learned, it uses げん instead. Just imagine someone clenching their fist at you. You gulp and say げん instead of けん out of fear.
父は、僕に拳骨を食らわせるふりをしました。
My dad pretended to hit me with his fists clenched.