1
general::
Force the cancellation or postponement of some event owing to bad weather. For example, Our picnic was rained out, but we hope to have it next week. [1920s]
American Heritage Idioms
2
general::
Phrase(s): rain something out
[for the weather] to spoil something by raining. • Oh, the weather looks awful. I hope it doesn’t rain the picnic out. • It’s starting to sprinkle now. Do you think it will rain out the ball game?
McGrawhill's American Idioms And Phrasal Verbs