A small dog possessing one or more of the following characteristics:
bulgy eyes,
squashed face, eyes that look in two or more directions at the same time, wandering lung, or comically vacuous expression. In short, any dog that arouses feelings of pity, revulsion, and the sneaking suspicion that it would have been kinder to have strangled it at birth.
E.g. pugs, Boston terriers, French bulldogs.
For the origin, do a google search for "hurr durr
derp derp derp"