What is a rain check? by Stephanie Vardavas
Answer by Stephanie Vardavas:
Before the era of electronic ticketing, if you bought a ticket to a professional baseball game it was a small strip of card stock, perforated in a couple of places. When you presented your ticket for entrance to the ballpark, about half of it would be torn off by the ticket taker and you would keep the rest. It would be marked with your seat location and other necessary information, including a section labeled RAIN CHECK.
In the event rain canceled the game before enough innings were played for it to become a legal game, you could present your rain check at the box office for a ticket to another game.
This is a rain check.
In any other usage, “rain check” is a metaphor, signifying that you have the right to come back on another day to receive the thing you could not get today. In a store it means “we are sold out of the item you want, so here is a slip of paper entitling you to buy one at the same price when we get more of it in stock.” In a social setting it is strictly oral and means, “I have a conflict so I am not available for the social event to which you have invited me, but I hope that you will invite me again some other time.”