The term is now applied to any bar or ingot of precious metals, and gold in particular. I have seen it suggested that the term was derived from the small buttons of gold or silver formed by the cupellation process. The Oxford Dictionary suggests that it comes from the French to boil, disappearance of the lead in the cupellation process could easily be thought to be because the lead was boiling off.
The term was also used to describe any small blob or lump of metal.