出恭去了 Off Somewhere to Relieve Himself
八戒笑道：「想是沙和尚帶師父那裡出恭去了。」說不了，只見沙僧來到。行者問道：「沙僧，師父那裡去了？」八戒笑道：「想是沙和尚带师父那里出恭去了。」说不了，只见沙僧来到。行者问道：「沙僧，师父那里去了？」
逐句解 Line by line
Bajie laughed and said, "I expect Sha Monk has taken the master off somewhere to relieve himself."
Trust Bajie to reach for the least alarming explanation available. 出恭 is a wonderfully prim euphemism — it started life as the wooden tablet an examination candidate carried when leaving the hall for the latrine, and the tablet's name became the deed's. Here 那 is nǎ, the vague somewhere-or-other, not the pointing this-one-there; the machinery would give the pointing reading, which is wrong at nine sites in this chapter.
Before the words were out, Sha Monk came into sight.
說不了, "the saying was not finished", is one of the novel's favourite hinges — somebody offers a comfortable theory and reality walks in on the next line. The second character 了 is liǎo, to bring to completion, and not the little grammatical marker that the same character carries a hundred and two other times in this chapter.
Pilgrim asked him, "Sha Monk, where has the master gone?"
Wukong's hope survives about eleven characters. This 那 is nǎ as well, but now as a straight question — where. Same character, same reading, and the difference between the two passages is only that Bajie was guessing and Wukong is asking.
The comic timing here is exact. Bajie produces a theory, and the novel refutes it in nine characters by simply having Sha Monk walk on alone.
Notice that the reader already knows the answer and the characters do not. The chapter is running on what a modern audience would call dramatic irony, and it will keep running on it for a long stretch — right through the business of the false head, which none of the three can see is false.