假親托意 A Feigned Kinship
那女子漸漸走近石邊，大聖躬身施禮，緩緩而言曰：「女菩薩何往？」那女子未曾觀看，聽得叫問，卻自擡頭。忽見大聖的相貌醜陋，老大心驚，欲退難退，欲行難行，只得戰兢兢，勉強答道：「你是何方來者？敢在此間問誰？」大聖沉思道：「我若說出取經求扇之事，恐這廝與牛王有親。且只以假親托意，來請魔王之言而答方可。」那女子渐渐走近石边，大圣躬身施礼，缓缓而言曰：「女菩萨何往？」那女子未曾观看，听得叫问，却自擡头。忽见大圣的相貌丑陋，老大心惊，欲退难退，欲行难行，只得战兢兢，勉强答道：「你是何方来者？敢在此间问谁？」大圣沉思道：「我若说出取经求扇之事，恐这厮与牛王有亲。且只以假亲托意，来请魔王之言而答方可。」
逐句解 Line by line
The woman drew gradually nearer to the rock. The Great Sage bowed low, made his salutation, and said in a slow voice, "Whither away, lady bodhisattva?"
菩薩 is pú sà, bodhisattva. Addressing a stranger on a mountain as one is pure flattery: she is nothing of the kind, and he has not the faintest idea who she is.
The woman had not been looking about her; hearing herself hailed and questioned, she raised her head.
未曾 is the literary negative for something that has never yet happened. 卻自 marks the movement as involuntary — her head comes up before she has decided to raise it.
Suddenly seeing how ugly the Great Sage's features were, she took a great fright. She would have drawn back and could not; she would have gone forward and could not; and so, all a-tremble, she forced herself to answer: "What place are you come from? How dare you question anyone here?"
相 is xiàng here, appearance. 勉強 takes qiǎng, to force oneself, not the qiáng of strength. And 欲退難退，欲行難行 is a four-part figure of paralysis, each half naming the wish and then cancelling it — she would go back and cannot, she would go on and cannot.
The Great Sage fell to thinking: "If I speak out the business of fetching scriptures and borrowing the fan, this creature may well be kin to the Bull King."
這廝 is contemptuous: creature, fellow, that thing. He is using it of a woman he addressed as a bodhisattva two sentences ago, and the gap between the two is the whole joke of the passage.
"Better to lean upon a feigned kinship for my pretext, and answer with the story that I am come to invite the Demon King."
假親 is a pretended relationship and 托意 is to rest your story on something. The relationship he means to pretend is the sworn brotherhood of five hundred years ago — which is not in fact a pretence at all, and which got him nothing at the Rākṣasī's door one chapter back.
The scene runs on a mismatch of manners. The Great Sage bows, makes a formal salutation, speaks slowly, and calls a stranger on a mountain a bodhisattva; she takes one look at his face and is too frightened to move in either direction. Two sentences later, thinking privately, he calls her 這廝 — that creature. The courtesy was entirely tactical, and the narrative shows us so immediately.
The four-part figure of her paralysis — she would withdraw and cannot, she would go on and cannot — is a stock formula, but it does real work here. She is not simply frightened; she is caught between two impossible movements, which is precisely where the Great Sage has put her by blocking the path with a greeting rather than a threat.
His decision is a small masterpiece of bad judgement. He resolves to hide the true errand and lean on a fake family connection instead — and the connection he has in mind, the sworn brotherhood with the Bull Demon King, is not fake. He tried exactly this at the Rākṣasī's door one chapter ago and came away with nothing. He is about to try it on the second wife, and the result will be considerably worse.