pack sand means: 1. The nautical expression. The tedious task of refilling ballast bags with sand bags is what gives rise to pack sand. Sand packing was performed in bilges on sailing vessels. The bilges held all kinds of wastes in the days before sail, including fecal matter and dead animals. See also pound sand 2. An insult to seafarers. A seafaring insult. (in Community Dictionary, added by Dominick Peters)