From Longman Dictionary of Contemporary Englishtake a rain check (on something)take a rain check (on something)informal especially American EnglishREJECT/NOT ACCEPT used to say that you will do something in the future but not now ‘Care for a drink?’ ‘I’ll take a rain check, thanks.’ → rain check
Examples from the Corpus
take a rain check (on something)• The warning voice could go take a rain check, she thought.• I'm sorry but I'm busy on Saturday - can I take a rain check? 