In Journey to the West, it contains stories about the monkey king. He commits various acts of mischief and chaos. Eventually, they decide to get together and do something about him. They call on the Buddha and ask him for advice. He goes to monkey and makes a bet over who can reach the edge of the universe first. Monkey reaches the edge of the universe, and he finds 5 pillars. He urinates on and writes his name on them. Then the buddha reveals himself and says those 5 pillars were actually his fingers and monkey never left his hand.
Was the urination of monkey perceived as impolite at the time when the story was written? Does writing his name on the pillars have any particular significance in that period?
It empahsis both his animalistic, childish behaviour with urination as well as his humanity and 'god hood' with his ability to write his own name. The story of Journey to the West, especially in the more modern takes of it, is about the duality of all of the characters, but especially the Monkey King in his ability to achieve enlightenment by overcoming his animal instincts.
Remember, the Journey is his penance, in the sense his release from Five Finger Mountain was paroled on the condition that he kept the Monk safe all the way to India.
EDIT: Don't just take my word on it; I only have a passing knowledge of the buddhist belief behind the folktale.