後門出頭 A Way Out At The Back
行者道：「這廝身體狼犺，窟穴窄小，斷然轉身不得，一定是個照直攛的，定有個後門出頭。你快去後門外攔住，等我在前門外打。」行者道：「这厮身体狼犺，窟穴窄小，断然转身不得，一定是个照直撺的，定有个后门出头。你快去后门外拦住，等我在前门外打。」
逐句解 Line by line
Pilgrim said, "This fellow's body is a lumbering great thing and the burrow is narrow. He certainly cannot turn round in there. He must be one that bolts straight through, so there is bound to be a back way out.
得 is the light de here, because the verb stands in front of 不得 — the "cannot manage to" pattern. 狼犺 is the elders' own word for the creature, coined back when nobody had seen it, and Pilgrim now has it in his mouth.
"Go quickly and hold the back door, and let me strike at the front."
Front door and back door: a snake's burrow described as a house, which is how this whole chapter keeps talking about the creature — a lane, a household, a back way out.
The reasoning is physical, and it is the first time in the chapter that anybody argues from the shape of a body to the shape of a plan. The creature is too big to turn round; therefore the burrow has two ends; therefore block one and strike at the other.
It also borrows the elders' word. 狼犺 was their term for a thing they had never laid eyes on, offered as a reason the party could not win. Pilgrim repeats it as the reason they will.