E. Cobham Brewer 18101897. Dictionary of Phrase and Fable. 1898.
means brown barrel. The barrels were browned to keep them from rusting. (Dutch, bus, a gun-barrel; Low German, büsse; Swedish, byssa. Our arquebus, blunderbuss.) In 1808 a process of browning was introduced, but this has, of course, nothing to do with the distinctive epithet. Probably Bess is a companion word to Bill. (See below.)