The name "Cash" (originally a surname) may come from a number of sources:
1. "Cash" is a form of "Case", an English occupational name for a maker of boxes or chests.
2. "Cash" is a shortened form of "Cashman", anglicized from the Gaelic: "Ó Ciosain", meaning "descendant of Ciosan" which my reference source states is a personal name perhaps derived from a diminutive of "ceas" (meaning "coracle"). For a definition of what a "coracle" is:
http://www.bartleby.com/61/49/C0634900.html
3. "Cash" is a shortened form of "Cashmore", an English habitation name taken from Cashmoor in Dorset, most likely derived from the Old English: "caerse" (meaning "cress") and "mor" (meaning "moor; marsh")
-- Nanaea