someone that you are partnered with, where you are each responsible for each others actions wether good or bad.
(from Southpark Season - 11, Ep. - 02 "Cartman Sucks")
also see - accountabilabuddyable
1)Butters - "Bradley!! dont jump! youre my accountabilabuddy! hows this gonna make me look??"
2) Counsellor - "this is just as much your fault Butters, Bradley is your accountabilabuddy, that makes you accountabilabuddyable!"