虎頭性子 The Idiot Cannot Hold Back
原來八戒在底下見他兩個戰到好處，忍不住掣鈀架風，跳將起去，望妖魔劈臉就築。那魔慌了。不知八戒是個虎頭性子，冒冒失失的諕人。原来八戒在底下见他两个战到好处，忍不住掣钯架风，跳将起去，望妖魔劈脸就筑。那魔慌了。不知八戒是个虎头性子，冒冒失失的諕人。
逐句解 Line by line
The fact is that Bajie, watching from below as the two of them fought to the best of it, could not hold back — he whipped out the rake, rode the wind up, sprang aloft, and jabbed straight at the demon's face.
忍不住 for the fifth and last time in this chapter, and this is the only one that belongs to Bajie. It gave away Wukong's disguise, produced his tears in the bottle, and made both him and the old fiend blurt out a gloss; here it puts the Idiot into a fight he was told to stay out of.
The fiend panicked.
Four characters. After twenty rounds of even fighting, one unexpected rake.
What he did not know was that Bajie has a tiger-head temperament, and startles people by sheer rashness.
諕 is xià, to frighten — the reading a dictionary does not offer first, and one this chapter has used eight times. 虎頭性子, a tiger-head disposition: all ferocious rush at the start. The narrator is explaining a tactical success by describing a character flaw.
Bajie disobeys and it works. He was posted to the sidelines before the duel began, he has been watching two better fighters for twenty rounds, and he cannot stand it any longer — and the demon, who held his own against the Great Sage, loses his nerve at the sight of a rake coming at his face.
The narrator's explanation is worth reading twice. Bajie did not frighten the demon by being formidable. He frightened him by being reckless, which is a different quality and one the book has spent seventy chapters treating as a failing. Here it is the only thing that has moved the fight all afternoon.