means The rear storage compartment of a car, or more broadly a large sturdy box for carrying or storing things — also the main woody stem of a tree, an elephant's nose, and the human torso.

from From the Latin 'truncus,' meaning the main stem of a tree or a body shorn of its limbs — the same root that gives us 'truncate.' That core idea of a solid central mass branched out wildly: the tree's trunk, the body's trunk, the big wooden chest 'trunk' (originally made from hollowed tree trunks), and the elephant's trunk (from its trunk-like thickness). The American car-trunk sense — where the British say 'boot' — kept the storage-box meaning rolling into the automobile age.