drag one's tail. 1. To be visibly sad or ashamed, especially in the way one moves about. “The image of a dog with its tail down, supposedly a sign of unhappiness,” as the Cassell's Dictionary of Slang puts it.

In George MacDonald's fantasy novel for children, The Princess and Curdie, he describes the scene in which Curdie introduces the Princess to a grotesquely ugly dog, Lina, who would accompany them on their quest:

She entered, creeping with downcast head, and dragging her tail over the floor behind her. Lina dropped flat on the floor, and covered her face with her two big paws. It went to the heart of the princess: in a moment she was on her knees beside her, stroking her ugly head and petting her all over.

Curdie tells the Princess that “Lina is a woman...she was naughty, but is now growing good,”which is a whole other story about women as dogs. The crucial thing here is to distinguish between one who is dragging his tail in this fashion or more deliberately as in the second definition.

2.To move or act with deliberate slowness.

This is an excellent description of how someone acts when they are considered to be lazy as a dog or when they are dogging it. While working like a dog is demanding, theorists suggest that if unrewarded for work and allowed to slack, dogs have no qualms about letting others toil. While I have little trouble imagining that dogs get away with whatever they can, I doubt that they engage in deliberate slowdowns, as humans might in labor actions less formal acts of resistance. To drag one's tail in this way strikes me as the province of those who are less eager to please and more likely to resent subservient status. Perhaps the comparison with dogs is a way of demeaning those who—enslaved by law or poverty—put as little effort into the job as possible because it is the only form of agency available to them. Placing a human in this category may justify paternalism. Those in subservient and service roles, like dogs, are perceived to need a firm hand. It may even make the boss feel a bit smug and superior, meting out a bit of discipline to a worker who is dragging his tail.

I am not convinced, however, that this is specifically a reference to the behavior of dogs. In many resources, this phrase is presented as a more polite way of saying “dragging one's ass,” suggesting that “tail” is a generic reference to rear end, bum, gluteus maximus, or what have you. And, while there is apparently an affliction in which dogs do drag their bottoms (a kind of canine constipation—don't ask) dogs do not drag their tails on the ground. At most they may lower them, as depicted in Figure 2.

Figure 2 shows a slowly moving dog. While he does not literally drag his tail, it low and his posture is dispirited or lazy. From Mixed Signals, the blog of Rob Ketcherside. Used with permission.