E. Cobham Brewer 18101897. Dictionary of Phrase and Fable. 1898.
A kind of halbert used by English foot-soldiers before muskets were employed. We find in the mediæval ballads the expressions, brown brand, brown sword, brown blade, etc. Sometimes the word rusty is substituted for brown, as in Chaucer: And in his side he had a rousty blade; which, being the god Mars, cannot mean a bad one. Keeping the weapons bright is a modern fashion; our forefathers preferred the honour of blood stains. Some say thè weapons were varnished with a brown varnish to prevent rust, and some affirm that one Brown was a famous maker of these instruments, and that Brown Bill is a phrase similar to Armstrong gun and Colts revolver. (See above.)
So, with a band of bowmen and of pikes,
Brown bills and targetiers.
Marlowe: Edward II. (1622.)
Brown also means shining (Dutch, brun), hence, My bonnie brown sword, brown as glass, etc., so that a brown bill might refer to the shining steel, and brown Bess to the bright barrel.