Rain Check Meaning

(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.

Example: 1877, Chicago Times, July 8, 1877:[1][3]
  The St. Louis club is the only nine in the league which gives its patrons the right to see a full game or no pay. In Chicago and other cities, after the first inning is interrupted by rain the spectators are supposed to have received their money’s worth. In St. Louis ‘rain checks’ are issued in such cases.
1896, Caduceus of Kappa Sigma, volume 11, p. 305
  [Regarding visiting the Indianapolis, Indiana chapter of the fraternity Kappa Sigma] … if in any respect the affair does not suit you—call at the box office and you’ll get your money back or get a rain check, just as you please.
  I can't go with you to the museum this Saturday, but can I take a rain check and go some other day?