Source: http://www.saiea.com/calabash/html/sadc.html
Many areas in the world have plants with hard skin, such as gourds. Once the inner flesh has dried out, these dried plants could be used as vessels, perhaps first to collect water. The name calabash is a generic English name for these dried out hard skins, and refers to many different plants that could be used for the same purpose.
Some time in the course of human history, like so many other objects, the calabash was used to make music. The vessel could be struck, or objects strung around it, such as the cabasa, to make shaker noises.
Source: http://www.saiea.com/calabash/html/sadc.html