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