The long pointed tooth at the corner of your own mouth. It is solid, it is useful, it is yours, and once it is gone it does not grow back. That is the whole picture the word carries, and it is also why the word turns up in a saying about wanting something badly.
可以想成嘴角那一顆又長又尖的牙。它是硬的、有用的、長在自己身上的，而且掉了不會再長回來。這個畫面就是整個詞的核心。也正因為它符合這幾個條件，英文才會拿它來講「我想要到願意拿自己身上的東西去換」。