A: I’m having a dinner party tomorrow. Can you come?

    B: Sorry, I’ll have to take a rain check, I already have plans.

    A: Okay, no problem!

    Note: In social interactions, this is a polite way to turn down an invitation, with the implication one is simply postponing it and that another time would be acceptable. The literal sense of rain check, which is an Americanism, is first found in the 1880s in reference to a baseball game. The practice of giving a rain check to a ticketholder was formalized in 1890 in the constitution of the National League. In other words, if it rained (something the purchaser had no influence over) and the game was postponed, the ticket holder could come back for another game.