Wordorigins.org

The the expressions go pound sand and not enough sense to pound sand are American slang from the 19th century. It is a reference menial, and often pointless, labor. From Alexander Smith’s 1857 City Poems:

Peopled now By outcasts, sullen men, bold girls who sat Pounding sand in the sun.

And we have this from the 26 December 1877 Globe of Atchison, Kansas:

We don’t know whether the young man you refer to knows enough to pound sand or not.

The latter phrase often appears in a longer form, not enough sense to pound sand down a rathole. This appears somewhat later. From the 1912 Dialect Notes:

He wouldn’t know enough to pound sand in a rat-hole; so don’t get him.

(Source: Oxford English Dictionary, 2nd & 3rd Editions)