Douglas Harper's Etymology Dictionary
also hangdog, 1670s, "befitting a hang-dog," a despicable, degraded fellow, so called either from notion of being fit only to hang a dog (see cutthroat) or of being a low person (i.e. dog) fit only for hanging. As a noun from 1680s.
Wiktionary
n. (alternative spelling of hang dog English)
Usage examples of "hang-dog".
Then sounding three blasts upon the silver horn which hung at his baldrick, Calverley led the way slowly on the road to Hasfield, Trollop following, with a hang-dog look, behind.
I, who am to be hanged this year, the nineteen-hundred-and-thirteenth after Christ, ask these questions of you who are assumably Christ's followers, of you whose hang-dogs are going to take me out and hide my face under a black cloth because they dare not look upon the horror they do to me while I yet live.
Look at the other figure alongside of him, his fist raised and with insults on his lips, with a hang-dog face, bloated with brandy, titular governor, official preceptor, and absolute master of this child, the cobbler Simon, malignant, foul mouthed, mean in every way, forcing him to become intoxicated, starving him, preventing him from sleeping, thrashing him, and who, obeying orders, instinctively visits on him all his brutality and corruption that he may pervert, degrade and deprave him.
He had held back in a hang-dog style when his son and Muscari had made a bold movement to break out of the brigand trap.