Verb
(idiomatic) To request or accept a rain check (an agreement from a merchant to honor a special offer, temporarily unavailable, after the expiration date).
(idiomatic, figuratively) In social interactions, to request a deferred invitation. To "take a rain check" is a polite way to turn down an engagement, usually with the implication that one is simply postponing it and another time might be acceptable.
I can't go with you to the museum this Saturday, but can I take a rain check?