The noun trunk refers to the main stem of a tree. If you want to make maple syrup, you need to tap the trunk of the maple tree and collect the tree's sap, which can then be boiled into a sticky syrup.
The torso of the human body, from the neck to the groin — but not including the head, neck, arms, or legs — is sometimes referred to as the trunk. If you have hives on your trunk, you probably itch on your back, chest and abdomen. The word trunk comes from the Old French word tronc, which referred to the trunk of a tree or a headless body.