Where did the phrase "rain check" originate?

This phrase originally referred to a voucher given to spectators at a baseball game that was rained out. The “rain check” allowed them to watch another game for free. Our sources indicate that the term came into being around 1884 and gradually came to refer to vouchers for other sports and eventually to vouchers you get at, say, the drugstore, when it runs out of sale-priced toothpaste.