Here are some definitions of rain check.
1. A rain check is a written promise from a seller to a buyer. It guarantees that a buyer can purchase a product for a certain price at a later date, usually because the item is out of stock.
2. A rain check is a ticket for future use given to spectators at an outdoor event, as a baseball game or concert, that has been postponed or interrupted by rain, (hence the name rain check).
Also, if you say you will take a rain check on an offer or suggestion, you mean that you do not want to accept it now, but you might accept it at another time.
