rain check: (idiomatic) In social interactions, a polite way to turn down an invitation, with the implication one is simply postponing it and that another time would be acceptable.

I can't go with you to the museum this Saturday, but can I take a rain check and go some other day?

Etymology:

The expression may have originated in the era of open-air markets where the occurrence of significant rain usually required a vendor to pack up their goods and leave. The term may also be based on the issuing of tickets to claim property such as a coat or hat check. Before 1889, US baseball fans were issued a new ticket if rain was heavy enough to cause a game to be postponed. Abner Powell added a detachable stub called a rain check that year. This quickly became a standard practice for all major league teams.