“Take a rain check”

p-3a answered your question: Idioms?

“Take a rain check”?

“Take a rain check?” means:

  • In principle I would like to do the thing you are suggesting
  • For some practical reason, I can’t do the thing now
  • I want you to know that I do actually want to do the thing
  • Can we do it at some yet-to-be-determined point in the future?

I think it’s a reference to a practice some stores have with sales:

  • Sometimes they will advertise a sale
  • And then run out of the thing that’s on sale
  • And give people who want to buy it rain check, which is a piece of paper entitling them to buy the product at the sale price once it comes back in stock
  • Most stores don’t do this anymore, but some do