一個長蛇 Such A Long Snake
八戒道：「原來是這般一個長蛇。若要吃人啊，一頓也得五百個，還不飽足。」行者道：「那軟柄槍乃是兩條信㮇。我們趕他軟了，從後打出去。」八戒道：「原来是这般一个长蛇。若要吃人啊，一顿也得五百个，还不饱足。」行者道：「那软柄枪乃是两条信㮇。我们赶他软了，从后打出去。」
逐句解 Line by line
Eight Rules said, "So it turns out to be a long snake of this kind.
長 is cháng. 原來 marks a realisation landing after the fact — the same phrase the old man used when he decided they were monks who cheat a man out of his dinner.
"If it means to eat people, it would want five hundred at a sitting and still not be filled."
得 is the full verb dé, "would need." The figure of five hundred is the number of households in the village, which Eight Rules is unlikely to have meant.
Pilgrim said, "That soft-shafted spear was its two forked tongue-tips.
The second character of 信㮇 is a rare form that many fonts will not render. It reads tiàn and the rhyme books gloss it as a slender rod; 信 is the snake's tongue. The weapon whose technique Eight Rules admired turns out not to have been a weapon.
"We shall chase him until he tires, and drive him out from behind."
軟 here means to go slack or tire — the same character that named the "soft-shafted" spear two segments ago, now doing something else entirely.
Eight Rules works out what the thing is and immediately prices it in people, which is how he has handled every quantity in this chapter. Five hundred at a sitting, and still hungry afterwards.
Pilgrim's correction closes the running joke about the spear. There was no shaft because there was no spear: what the two of them fought all night was a forked tongue. Eight Rules had praised its technique and ruled out two named schools before settling on a third name of his own invention.
The plan that follows takes eleven characters and is the whole strategy for the rest of the chapter — drive it from behind, out through the front.