To the intellectual bad ass it is a more effective and more offensive way to say someone is a jerk. Jerkwagons tend to seem nice at first, but then turn around to slap you in the face...with their wagon, that they carry all their bullshit in.
Person 1: Hey how's it goin??
Person 2: Pretty good, haven't seen you for a while! You?
Person 1: Same, hey want to hang out later?
*Person 2 is offline*
Person 1: Wow, what a jerkwagon.
Harry: Bellatrix killed Sirius! My last living family!! *cries*
Ron: Sorry mate, that bitch is a jerkwagon.