rain on someone's parade
verb phrase (figurative)

Verb. rain on someone's parade. To disappoint or discourage someone by ruining or criticising their plans or aspirations.

Prevent someone from enjoying an event; spoil someone's plans.

Oxford Dictionaries

---
ORIGIN

Sometimes popular songs or musicals trigger new words or phrases into the language.

"Don't Rain on My Parade" is a popular song from the 1964 musical Funny Girl. It was also featured in the 1968 movie version of the musical. Both the movie and stage versions feature Barbra Streisand performing the song. Here's a snippet:

"Don't tell me not to live
Just sit and putter
Life's candy and the sun's
A ball of butter
DON'T bring around a cloud
To RAIN ON MY PARADE"

Song Lyrics from the 1964 musical Funny Girl

And you can hear it here: http://bit.ly/2EIlZHc

---
SYNONYMS
I hate to:... queer your pitch, spoil the fun, put a dampener on proceedings

---
Practice OWAD in a conversation today. Say something like:

"I hate to RAIN ON YOUR PARADE, but lots of people have tried that strategy and it never worked."