a buzz kill
Someone or something that makes people less happy, excited, or enthusiastic about some event or thing. Primarily heard in US. I was so excited to see the concert, but all the technical problems with the lights and sound were such buzzkills. I'm sorry to be a buzzkill, everyone, but we've received a number of complaints from neighbors about the noise, so we'll have to turn down the music.
Farlex Dictionary of Idioms. © 2015 Farlex, Inc, all rights reserved.
a buzz killAMERICAN, INFORMAL
If a person or something that happens is a buzz kill, they spoil an event by making people less happy and excited. Sorry I've been a buzz kill tonight. They discovered his money had been stolen. Talk about a buzz kill.
Collins COBUILD Idioms Dictionary, 3rd ed. © HarperCollins Publishers 2012