Back Of One's Hand
noun
Your hand's turtle shell is the back of it (see you can make it shaped like a turtle shell if you try hard!). That's why this is the back of one's hand.
てのこう
The readings are the respective vocab readings of 手 and 甲.
とても綺麗な手の甲を持っていることが彼女の自慢です。
She is proud that the backs of her hands are so beautiful.