Well, to the best of my limited knowledge this could refer to two things which are actually the same. On a watch case...which is gold filled, brassing refers to where the gold fill or overlay has worn through from polishing exposing the brass that the case is actually made out of...brassing...

This can also refer to the parts of a watch movement which are made of brass and flashed, or plated, in nickel or gold or possibly any other metal that is used for plating. With multiple cleanings, or when the parts have been left in the cleaning solution too long, or because of necessary "over" cleaning to remove heavy rust and corrosion... the plating can come off and expose the brass that the plates are made of...brassing. This type rarely effects the performance of the watch to any great degree but it may hint at its history.

Hope this helps.