In Gulliver's travels part 3, the tradition to lick the dust before the foot stool is to humiliate the human beings and also to take revenge,Explain.

This was a regular custom........ and adhered to be everyone wishing to seek admittance. The floor was actually made impeccably clean before Gulliver was made to;

"....... crawl upon my belly, and lick the floor as I advanced; but, on account of my being a stranger, care was taken to have it made so clean, that the dust was not offensive."

For someone the King is unhappy with, the floor might be dirtied on purpose;

".......sometimes the floor is strewed with dust on purpose, when the person to be admitted happens to have powerful enemies at court; and I have seen a great lord with his mouth so crammed, that when he had crept to the proper distance from the throne; he was not able to speak a word."

As far as revenge, another form of dirtying took place;

".....when the king has a mind to put any of his nobles to death in a gentle indulgent manner, he commands the floor to be strewed with a certain brown powder of a deadly composition, which being licked up, infallibly kills him in twenty-four hours."