It is what button-collectors call a "Fashion" (for "fashionable clothing") button with a tinplated iron or steel back. It was most likely on a "sportscoat" or a (cardplayer's)vest. Because its iron back is corroded enough to prohibit reading any maker's-mark which might have been on it, we cannot narrowly time-date it for you. It is most likely from the early-through-mid-20th-Century.
Despite being called a Fashion button, an iron/steelbacked buttos would not be worn on expensive clothing, because the tinplating eventually wears off, which means the iron/steel back will afterward be making rust stains whenever the clothing gets wet (or washed).