Matron Head large cent? Color and size makes sense.
to chop off a chunk of a george III halfpenny would take some doing as they tended to be pretty sturdy coins and not thin/brittle. Makes me wonder if your piece is a button or maybe a jetton.
I'm assuming a New England area find? Nuremberg jettons are far more plentiful finds in the vicinity of Jamestown, VA but that they would make it up to 17th century New England would also be logical. Another more exciting possibility for very thin, brittle brass piece (besides jetton or button) would be a Somers Island 'Hog Money' coinage piece--- however, keep in mind those are quite rare. But they are of the early 1600s period and maybe a piece or two sailed up from Bermuda.