絕後之計 A Scheme to Leave No Fruit Behind
二仙童問得是實，越加毀罵。就恨得個大聖鋼牙咬響，火眼睜圓，把條金箍棒揝了又揝，忍了又忍道：「這童子只說當面打人也罷，受他些氣兒。送他個絕後計，教他大家都吃不成。」好行者，把腦後的毫毛拔了一根，吹口仙氣，叫：「變！」變做個假行者，跟定唐僧，陪著悟能、悟淨，忍受著道童嚷罵。二仙童问得是实，越加毁骂。就恨得个大圣钢牙咬响，火眼睁圆，把条金箍棒揝了又揝，忍了又忍道：「这童子只说当面打人也罢，受他些气儿。送他个绝后计，教他大家都吃不成。」好行者，把脑后的毫毛拔了一根，吹口仙气，叫：「变！」变做个假行者，跟定唐僧，陪着悟能、悟净，忍受着道童嚷骂。
逐句解 Line by line
The two immortal lads, having questioned them and got the truth, reviled them all the more.
Getting the truth does not calm them; it does the opposite. 問得是實, wèn de shì shí, is "questioned them and found it true," with the linking 得, de. 毀罵, huǐ mà, is to pile on abuse.
It so galled the Great Sage that his steel teeth ground and rang and his fiery eyes glared round. He clenched and re-clenched the gold-banded cudgel, and, swallowing his temper again and again, thought: "These striplings do nothing but say I would strike a man to his face. Well and good—I shall put up with a measure of their abuse,
Grinding teeth, blazing eyes, and a grip that tightens and tightens on the cudgel: the narrator gives the whole build-up of a temper being held down. 大聖, Dàshèng, "the Great Sage," is short for the title Wukong once awarded himself, 齊天大聖, "Great Sage Equal to Heaven." 金箍棒, jīn gū bàng, is the gold-banded iron cudgel. 當面 is dāng miàn, "to your face"; 氣兒 takes the diminutive suffix 兒, ér — not abuse in general, just a helping of it.
"—and deal them a scheme to leave no seed behind, so that none of them, great or small, shall ever eat of it again."
絕後 is about the bleakest thing you can wish on a family in classical Chinese: to be left without descendants. A 絕後計, jué hòu jì, is a plan that cuts off the line, and Wukong means it exactly — kill the tree and there is no fruit for anyone, ever again. 教 here is jiào, "to make [someone do]," not jiāo, "to teach." 吃不成, chī bù chéng, is "cannot manage to eat."
Good Pilgrim! He plucked a hair from the back of his head, blew on it a breath of immortal air, and cried, "Change!"
好行者, "Good Pilgrim!" is the storyteller clapping his hands before the trick starts, a cue that something worth watching is about to happen. 毫毛, háo máo, are the fine body-hairs Wukong plucks and turns into whatever he needs; it is his signature, and a Ming audience knew it as well as a modern one knows a superhero's catchphrase.
It changed into a false Pilgrim, which kept close by the Tang Monk, attending Wuneng and Wujing, and bore the lads' railing in his place.
The hair becomes a 假行者, a false Pilgrim, who sits and takes the scolding while the real one slips away — an alibi that nods along in your place. 悟能, Wùnéng, and 悟淨, Wùjìng, are the religious names Bajie and Wujing received on joining the pilgrimage. Both instances of 著 here are zhe, marking action still in progress: attending, enduring.
Wukong's restraint is itself the plot. Rather than brawl with two boys, he sits on his temper and calculates something subtler and far crueller: destroy the tree, so that the loss can never be made good. Everything from here to the end of the chapter follows from that decision.
The phrase for it is 絕後計, a "scheme to leave no heir," and the choice of words matters. A quarrel over four pieces of fruit is being escalated into an act against a lineage — which is precisely how the abbey will read it, and why its master will not simply let the pilgrims go.
The plucked hair is a stock marvel, but it is doing careful work here. The false Pilgrim absorbs the boys' abuse in full view while the true body leaves unseen, so the monkey manages to be dutiful and destructive at the same time. Watch for the same trick twice more before the chapter closes, once with willows and once with a stone lion.