Noun
てのこう
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.
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.
The readings are the respective vocab readings of 手 and 甲.