I'll Have To Give You A Rain Check On That

( give a promise to do that thing at a future time ... )

To get a rain check is to receive a promise to do some thing at better time, such as when it is not raining. Example: "He was going to take me to the park, but it rained, so he gave me a rain check."

Some things cannot be done when it is raining, so we give a person a rain check which means we give that person a promise to do that thing in the future when it is not raining. Example: "Sorry, but I can't go today. I'll have to give you a rain check?"

A rain check is a promise to do it at a better time, when the weather is good. Example: "I asked her to go out on a date, but it turns out she has a boyfriend." Reply: "I hope she gave you a rain check?"