A downsack is some one who does not like to go out and do anything. They will also bring other people down to make them feel better about them self.
"Is connor coming to the party this weekend?"

"nahh he's being a complete downsack saying that the party's going to be absolutely shit, so he's staying at home again"

"what a downsack!"
by Supersickseal June 26, 2012
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