反了陰陽 Yin And Yang Turned Around
八戒笑道：「不羞，你這反了陰陽的。他二位老媽媽兒，不叫他做婆婆、奶奶，倒叫他做公公？」眾人道：「莫弄嘴，快尋你師兄去。」八戒笑道：「不羞，你这反了阴阳的。他二位老妈妈儿，不叫他做婆婆、奶奶，倒叫他做公公？」众人道：「莫弄嘴，快寻你师兄去。」
逐句解 Line by line
Eight Rules laughed. "Shame on you! You have got your yin and yang the wrong way round.
反了陰陽 — the two principles the wrong way round — is the crude engine of the joke, and it does in four characters what English would need something coarser for.
"These two are old dames. Instead of calling them goodwife or grandmother, you call them grandfather?"
兒 is the weak little er on 媽媽兒; of the chapter's seven occurrences only 兒子, a son, keeps the full tone. 倒 is dào, the adverb that flags a contrary turn — you go and do the opposite. Three more doubled terms of address stand in this one sentence, all weak on the second syllable.
The crowd said, "Stop wagging your tongue and go and find your brother."
弄嘴 is to make play with the mouth, and it turns on the same 弄 he used a moment ago for what his brother did to him. The crowd is telling him to stop doing with words what Pilgrim did with a whirlwind.
Eight Rules is not apologising; he is insisting. Told he has got the sex of two court officials wrong, he replies that they are old women and the mistake is everybody else's — which is a joke about what a eunuch is, made to a eunuch's face, by a man being held for theft.
The chapter has packed its whole stock of kinship words into this exchange. Grandmother, grandfather, goodwife, mother — four kinds of relative in two sentences, every one of them misapplied, and every one doubled, so that the rhythm of the joke is a sort of babble.
莫弄嘴 ends it flatly. Nobody laughs and nobody takes offence; the crowd simply wants to get on, and Eight Rules's best material has landed on people with somewhere to be.