I believe this started in the US. If you went to a baseball match (or similar out door event) and after paying your admission it was cancelled (say, due to heavy rain), you would be given a raincheck to let you in next time. The problem is, if this was done in the UK, it would be spelt raincheque, as it is a sort of promissary note. Perhaps one of our US contributors could confirm (or put lots of egg on my face by denying) this.