狐假虎威 The Old Man Has Vanished
正行間，不見了那報信的老者。沙僧道：「他就是妖怪，故意狐假虎威的來傳報，恐諕我們哩。」行者道：「不要忙，等我去看看。」正行间，不见了那报信的老者。沙僧道：「他就是妖怪，故意狐假虎威的来传报，恐諕我们哩。」行者道：「不要忙，等我去看看。」
逐句解 Line by line
As they were going along, the old man who had brought the news was nowhere to be seen.
正行間 is in the very act of travelling — the formula this book uses to hinge one scene onto the next. Twelve characters, and the chapter's most important character has quietly left the stage.
Sha Monk said, "He must have been a demon himself, coming to bring word on purpose, borrowing the tiger's terror like the fox, to frighten us."
狐假虎威 is one of the best-known set phrases in the language: the fox that walks in front of the tiger so the other animals scatter, and takes the credit. It comes from a Warring States story and is still exactly what you say about somebody trading on borrowed authority. 諕 is xià, to frighten — one of the readings the plain machinery gets wrong every time.
Pilgrim said, "Do not be in a hurry. Let me go and have a look."
看看 doubled softens the verb into just a look, no undertaking. It is the same casualness he brought to the first interview.
The vanishing is the chapter's quietest and most important beat. An old man who shouted a warning, was knocked down twice, gave a full inventory of an enemy army — and is simply not there any more.
Sha Monk's explanation is the wrong one and it is the reasonable one. A stranger who appears with terrifying news and then disappears was probably part of the terrifying news. The idiom he reaches for is about exactly that: borrowed menace.
It is worth noticing that the second and third disciples have now both spoken more sense than Pilgrim has all chapter, and both have been ignored.