橫行亂走 Walking Sideways Among the Guests
大聖忽作人言，叫：「饒命，饒命。」老龍道：「你是那裡來的野蟹？怎麼敢上廳堂，在尊客之前，橫行亂走？快早供來，免汝死罪。」大圣忽作人言，叫：「饶命，饶命。」老龙道：「你是那里来的野蟹？怎么敢上厅堂，在尊客之前，横行乱走？快早供来，免汝死罪。」
逐句解 Line by line
The Great Sage suddenly spoke with a human voice and cried, "Spare my life, spare my life!"
忽作人言 — suddenly spoke in human speech — is the standard formula for an animal shape breaking into language, and it is the first thing that marks this crab as not a crab.
The old dragon said, "Where are you come from, wild crab?"
那裡 is a question here — from where — so 那 is nǎ, not the pointing nà.
"How dare you come up into the hall and go sidling and blundering about before an honoured guest?"
Here is the pun the next two passages are built on. 橫行 means literally to go crosswise, which is what a crab does, and figuratively to throw your weight about lawlessly, which is what a bandit does — modern Chinese still uses it in the second sense. The old dragon means the second. The prisoner is about to claim the first.
"Confess quickly, and I will remit the capital charge."
供 is gòng here, to depose or give evidence — a courtroom word — not the gōng of supplying that the same character took at 年供柴，月供米 earlier in the chapter, where the Jade-Faced Princess was itemising her outgoings.
The scene is played from start to finish as a court hearing. The old dragon does not ask what the crab is but where it comes from and how it dares; he charges it with an offence against decorum committed in front of an honoured guest; and he demands a deposition on pain of death. The submarine palace turns out to have a magistrate's bench as well as a banqueting hall, and to be able to switch from one to the other between courses.
The charge is 橫行, and the word is doing double duty. It is the ordinary term for moving crosswise, which is exactly how a crab walks, and it is also the standing idiom for behaving lawlessly. The old dragon has accused the creature of both at once without noticing, and the Great Sage is about to build his entire defence on the ambiguity.