"meaner than a junkyard dog" is a common phrase to refer to someone who will readily attack at the slightest provocation. It comes from large dogs (Doberman's, German Shepherds, part wolf dogs) allowed to run free at night in junkyards to prevent theft of the auto parts in a large yard.
It's a little more serious than the teenagers messing around phrase implies. It also refers to female dogs who will defend their pups. Sort of equivalent with messing with a mama kangaroo.
Artemis
