A downer is a person or a message that puts you down, your energy down or your positiveness down.
You might say:
“Whaou! That was a great evening! I had a great time”
Their answer could be:
“What? What did you like about it?”
You might say:
“Whaou! That was a great evening! I had a great time”
Their answer could be:
“What? What did you like about it?”