Below is the quote from the text explaining how the Lilliputians describe Gulliver's hat.
". . . a messenger arrived to tell his Majesty that some of his subjects, riding near where I was found, had seen a great black substance lying on the ground. It had wide, round edges, rising up in the middles as high as a man. It was not a living creature, as they thought at first. It did not move. Some of them walked around it several times, and climbing to the top and stomping on it, they found it was hollow. They thought it might be something belonging to the Man Mountain."
They describe this hat with a completely blank slate. They have no idea what they are about to encounter, in fact they wonder at first if it is alive. The describe the edges and the middle of the hat the way it LITERALLY appears and cannot understand the purpose of such a thing--perhaps because it is so very large to them.