留個名罷 Leaving a Name
行者坐在上面，聽見說出這話兒來，已此識破了，道：「我弄個手段，索性留個名罷。」大叫云：行者坐在上面，听见说出这话儿来，已此识破了，道：「我弄个手段，索性留个名罢。」大叫云：
逐句解 Line by line
Pilgrim, seated up above and hearing these words spoken out, saw the game was up and said, “Let me work a little trick — I may as well leave them a name.”
話兒: 兒 = er. 已此識破 "by now the trick had been seen through." 索性留個名罷: 罷 = ba, the softener at the end of a sentence; 索性 is "might as well, and be done with it." He is not cornered -- he has decided to sign his work.
He cried out in a loud voice:
大叫云 "he called out aloud" -- the tag that introduces the taunt in verse.
“You reverend Daoists, you Daoists — how foolish are your fancies!
道號道號 "O Daoist reverences, Daoist reverences" -- calling them by their titles and making the titles ridiculous. 你好胡思: 好 = hǎo, "how, so"; 胡思 "wild, groundless notions."
What Three Pure Ones would ever stoop to this mortal ground?
那個: 那 = nǎ, the questioning reading -- "which Three Pure Ones?" 肯降凡基: 降 = jiàng, to stoop down to the mortal ground.
I'll take my true surname and make it known to you:
吾將真姓: 將 = jiāng, taking hold of the thing named -- "I take my true surname and..."
the monks of Great Tang, come west by imperial command.
大唐僧眾 "the company of monks from Great Tang." 奉旨 "by imperial command."
On a fine night, with nothing to do, we came down to your temple halls,
良宵無事 "a fine night, nothing to do." 下降宮闈: 降 = jiàng, came down to the temple chambers. He is parodying the language of a divine visitation while describing a burglary.
ate up your offerings, and sat about idle and amused.
吃了供養: 供 = gòng, the offerings. 閑坐嬉嬉 "sat about idle, laughing."
You have knocked your heads to us in worship — how are we to repay it?
蒙你叩拜 "favoured with your kowtows." 何以答之 "and how shall we repay it?" -- the last courtesy before the punchline.
What holy water was that, then? What you drank was every drop my piss!”
那裡是甚麼聖水: 那 = nǎ, the questioning reading again, and 甚 = shén in 甚麼, "what" -- worth setting beside 不甚 a few lines back, where the same character is shèn and means "very." 都是: 都 = dōu. 一溺之尿: the falling tone of 溺 pulls 一 to yí.
Wukong ends the trick by confessing it in verse -- and in the same tidy four-character parallel lines the immortals used for their prayers. The form that carried their reverence now carries his jeer, which is a neater piece of mockery than anything he could have said in prose.
He observes every courtesy on the way down. What Three Pure Ones would come to a place like this? Let me tell you my real name. We came down on a quiet evening, ate your food, and sat about enjoying ourselves. You have bowed to us -- how shall we repay you? And then the last line, which repays them.
The passage also puts the two readings of 甚 within a few lines of each other: 不甚 "not very," where it is shèn, and 甚麼 "what," where it is shén. The same character doing two unrelated jobs, in one small scene.