If it's very thin but hard (not foil-like) copper or brass— and if there is no attachment point on the back, where a shank/eye has broken off— then it may be the die-stamped "face" for a three-piece gilt dress button, with the front and back held together by a separate rim, as on "staff" buttons. Also, some wooden and composition dress buttons had medallic or embossed disc inserts; so, it could be from one of those.