In Chapter 5 of George Orwell's Animal Farm, a debate between Napoleon and Snowball takes place which results in Snowball being chased away from the farm by Napoleon's guard dogs. After Snowball's departure, Napoleon declares an end to any further such debates and, because Napoleon himself is not very eloquent, sends Squealer around to provide additional explanation for Napoleon's actions. Squealer tells the animals that they must be disciplined and obedient to Napoleon or else their enemies, the humans, will come back and retake the farm. Squealer asks them, "Surely, comrades, you do not want Jones back?"
Squealer's question is immediately followed by the narrator's comment: "Once again this argument was unanswerable." In theory, no argument should be unanswerable, but in reality the animals would be wise not to answer if they valued their lives. The physical violence which Napoleon had used against Snowball could just as easily be used against them. If the animals had responded, "Yes, we want Jones back", then surely Napoleon would have had them threatened with physical violence.
Also, a few paragraphs earlier, the narrator tells us that the animals were troubled by Napoleon's new decrees. Orwell comments that "Several of them would have protested if they could have found the right arguments." Thus, Orwell suggests that the other animals are not clever enough to argue against Napoleon's declarations. Throughout the novel, Orwell mentions the difficulty that most of the animals have in learning or remembering various things. If most animals are unable to learn or remember basic facts, then how can we expect them to make the complex sort of arguments required to answer Squealer successfully.
Chapter 5 sees the expulsion of Snowball and Napoleon's rise to military ruler instead of co-equal leader. When the other animals question Napoleon's motives, Squealer explains that Napoleon has known for some time that Snowball is a traitor to the farm, that he has been selling secrets to the farmers, and that he plotted to kill Napoleon and take control (just like Napoleon actually tried to do). He finishes with his classic appeal to fear:
"One false step, and our enemies would be upon us. Surely, comrades, you do not want Jones back?"
Once again this argument was unanswerable. Certainly the animals did not want Jones back... Boxer, who had now had time to think things over, voiced the general feeling by saying: "If Comrade Napoleon says it, it must be right."
(Orwell, Animal Farm, george-orwell.org)
The argument is "unanswerable" because of the implicit notion that arguing with Napoleon will bring Jones back to the farm. This is not true, but because Squealer repeats the argument so often, the animals have begun to believe it. By refusing to think through the problems and see how Napoleon is actually doing what he accuses Snowball of plotting, the animals prevent themselves from coming to a sane answer for Squealer's argument; essentially, it is unanswerable because they literally cannot see past it, preferring, as Boxer does, to have blind faith in Napoleon.