不知好歹 You Do Not Know Better
那老兒笑道：「你這小和尚年幼，不知好歹，言不幫襯。那妖魔神通廣大得緊，怎敢就說貶解他起身？」那老儿笑道：「你这小和尚年幼，不知好歹，言不帮衬。那妖魔神通广大得紧，怎敢就说贬解他起身？」
逐句解 Line by line
The old fellow laughed. "You are a young little monk who cannot tell good from bad, and your talk does you no good at all.
不知好歹 is still current and still an insult — not knowing what is good for you, unable to tell a kindness from an injury. 幫襯 is a fine old word for backing somebody up: 襯 is the lining of a garment, the layer underneath that holds the shape, so to 幫襯 somebody is to be that for them. 言不幫襯 therefore means roughly that your words are doing nothing to hold you up.
That demon's powers are enormously great. How dare you talk about banishing him and packing him off?"
神通 is a Buddhist technical term before it is a compliment: the supernatural faculties a practitioner acquires, six of them in the standard list. 得緊 is the degree complement — great, and how great? — very — with 得 going unstressed as de, exactly as it did in 生得俊雅 a moment ago.
The old man's rebuke is aimed squarely at the disguise, which means the disguise is still working. He is not saying you are a monkey. He is saying you are a child who has no idea what he is talking about.
And he is right about the substance while being entirely wrong about the speaker. The demon's powers are enormous; talking about banishment is absurd. The one thing he cannot know is that the pretty boy in front of him is the only creature on the mountain for whom it is not absurd at all.