西海龍身 A Dragon Of The Western Ocean
行者道：「你不知就裡。我那馬不是凡馬，他本是西海龍身。若得他肯去便溺，憑你何疾，服之即愈。但急不可得耳。」行者道：「你不知就里。我那马不是凡马，他本是西海龙身。若得他肯去便溺，凭你何疾，服之即愈。但急不可得耳。」
逐句解 Line by line
Pilgrim said, "You do not know what is behind it.
就裡 is the inside of a matter — what is really going on. For the fourth time tonight an objection is answered by telling the objector he does not understand, and this time something is actually explained.
"That horse of mine is no ordinary horse. He was originally a dragon of the Western Ocean.
The white horse's origin has been sitting unused for a long stretch of the pilgrimage, and it is raised here to justify a prescription. 那 is nà, that.
"If he can be got to void, then whatever your sickness, you are cured the moment you take it. Only it is not to be had in a hurry."
Dragon urine cures whatever it touches, which turns the most absurd ingredient in the pills into the most powerful one. 溺 is niào here, urine, not the nì of drowning, and 便 keeps biàn — both readings belong to the medical register. Both occurrences of 得 are the full verb dé, to get hold of.
For the fourth time an objection is met with "you do not understand" — but this time something is actually explained. The white horse is a dragon, and dragon urine cures whatever it touches, which converts the most absurd ingredient in the prescription into its most powerful one.
The horse's origin has been sitting unused for a long stretch of the pilgrimage. Bringing it out here, to license a prescription, is characteristic of how this chapter works: everything it reaches for is real, and everything it does with it is ridiculous.
"Only it is not to be had in a hurry" is the line that turns the scene. Pilgrim knows the difficulty before Eight Rules meets it, mentions it in half a breath, and sends him off anyway.