sandbag

To pile-on to a person, or to join onto a side of an argument. The most common uses of sandbags are in sports arguments and in roast sessions with the boys.

Sam: Brad you are a dumb dufus

Brad: Sam you make no sense dude.
Trent: Yeah Sam he's right you eat bro.

Sam: Why do you gotta sandbag me like that Trent?