Another way of saying 'get lost', 'go away' and generally 'don't bother me'. Obviously, it is similar to pound sand
, but if it is a corruption of that term, or something else entirely, maybe somebody else knows. I'm not sure why anyone would pound salt down a rat hole, but maybe it is something that was done at one time. Perhaps it refers to crushing large pieces of salt into finer crystals, or weighing salt into smaller increments; both of which would be dull menial work. Maybe it refers to walking on salt, similar to 'hit the bricks' or 'kick mud', since salt is often spread on the ground as a de-icer in winter and is also common in desert and seashore areas.
When I brought the broken chair in for warranty replacement, the manager basically told me to pound salt, it wasn't a warranty problem.