Rain...it's never nice...I expect some at any moment.
It means, basically, to be the bearer of bad news, or to mess things up for someone! It's sometimes used in an apologetic sense, i.e., someone's sorry for being the bearer of bad news.
Use in Normal SpeechEdit
When bringing bad news.
Anna: "Yess! I've just nailed the pronunciation of another tricky surname!"
Lulu: "I don't want to rain on your parade...but the owner of that surname has just been mullered..."
When saying someone's messed things up.
"Why thank you, Liam, you've really rained on my parade. Things just get better and better..."