A trunk, also called a chest, blanket box or toy box depending on what it is used for, is a large box usually made out of wood that is used for storing things in. The most famous version of the trunk is the treasure chest, infamously used by pirates to bury their treasure in. The reason a car trunk has the name it does is because early cars used to have an actual trunk on the back, which gradually evolved to the trunk we know today.
A car's trunk, also called a boot in British English, is the space at the back of the car used to carry things like luggage. Older cars used to have an actual trunk at the back, which is why a car's trunk has the name it does.
A tree's trunk is the main supporting structure of the tree. Tree trunks are covered with bark and branches grow out from the sides of the tree's trunk. As the tree gets older its trunk gets thicker. Wood for making furniture or houses out of is cut from the trunk of a tree.