The English word Cash is of the French caisse, itself a borrowing of the Provenal caissa. That Provenal word is a derivative of the Latin capsa (box, chest), most likely by way of an unattested Vulgar Latin form *capsea; Spanish caja and Portuguese caixa are their respective languages' reflexes.
From the original sense of a box or a chest, the word came to refer to a sum of money such as was or might be contained in one, and eventually to specie or, with the elimination of metallic standards, banknotes. In this sense, it is used in contrast to credit or other financial instruments.
The word "cash" can also be traced back to: Sanskrit karsa, a weight of gold or silver but akin to Old Persian karsha-, a weight. a unit of value equivalent to one cash coin.