"Go Pound Salt"

It's driving me crazy! What is the orgin of this phrase? "Go Pound Salt." I've heard it used primarily
in the same way you would say "Get Lost" or "Go play on the Freeway." Were there salt pounders?
Was this an endless task? Please email me if you know... thanks,

The longer form is "go pound salt up your ass" which sounds decidely unpleasant. Why salt? Why "pound"? I haven't a clue.

Becausing "pounding" (rather than gently packing) would add to the pain of salt insertion. Like pouring salt on someone's wounds only more so.