As he heard Gulliver out, his master’s face showed great unease. Doubting, or not believing, is so rare in that country that people there do not know how to behave when it happens.
Gulliver often talked with his master about human nature in other parts of the world. At times he had occasion to mention lying and false representation. Though the master’s judgement was otherwise very sharp, he had great difficulty understanding what Gulliver meant.
The Houyhnhnms’ language has no word for lying or falsehood. Instead they say someone has said the thing which was not.