although more aggressive, as the act of "pounding sand" is a useless endeavor. The rat will eventually dig its way out.

It implies that the object of the insult is stupid enough to engage in a difficult, useless activity; which even then, is a better use of their time than what they are doing now. That's the original meaning of the idiom, but the phrase's meaning has broadened since then.

There. Now, if you didn't like my explanation, then go pound sand.