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.

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.