put a downer on (something)
To inhibit something or make it less pleasant or enjoyable; to have a subduing or deadening effect on something. The foul weather really put a downer on our picnic yesterday. Without question, this tax scandal has put a downer on the senator's likelihood of being re-elected. The construction happening near our building has really put a downer on sales this month.
put a downer on something
If something or someone puts a downer on an event, they make it less enjoyable. The argument put a downer on our school holiday break.