Definition of pound sand:
(v) :
(slang, idiomatic, derogatory, dismissal) To go away; get lost; go to hell; to waste one's own time (with some other meaningless task) rather than waste the speaker's time with something equally irritating or pointless.
(v) :
(informal, idiomatic) To engage in a futile activity.
(v) :
(informal, idiomatic) To engage in a simple, useless activity that anyone would reasonably know how to perform.