Someone who takes a last minute, unwanted and dumb stand in an arguement, debate or row
Dude... She's being such a donkey sitter.
She's being the world's worst donkey sitter by being on Alyssa's side
Donkey sitter is actually a person who is a fence sitter but suddenly decides to take a stupid stand at the 11th hour unwittingly, which is uncalled for.
Vernon was a donkey sitter in the Micheal and Denzil controversy