In the DJ category NOT OF GOOD CHARACTER for $800: "One who speaks ill of you when you're not around, or who implants a dagger in your dorsal region"
Nothing specifically says that they're looking for a one-word response, so my guess is that a phrase that includes 'stabs you in the back' would be OK, though why you would come up with an awkward phrase instead of the one-word term is beyond me. Still, we're not the judges and this is definitely a judgment call, so you're not going to get a definitive answer here.