Before the advent of the space program, various parties tried to explain what the moon and other planets consisted of and, how, or if, they were peopled. Explanations can be found in documents ascending from antiquity to modern times. It sounds ridiculous to us that anyone could believe that the moon is anything more than a large rock, but of what did people believe the moon consisted?
Green cheese seems a little far fetched, but lets look at the data. It is round like most cheeses are when they are newly minted. It has an uneven surface. The term “green” may mean color to us, but the intent was the newness not the hue. There is also the distance factor. As far away as the moon is from us, anybody’s guess as to its make up should not be disregarded. So when they looked up they saw a fresh cheese wheel hanging in the sky. We could also make a case for more improvident guesses based on what we see of the moon. Sometimes we have a bright full moon, or a slim fingernail. The argument could be made that there are several different moons and that they rotate the Earth at the same speed and show up at different times of the month. A legend could be started that some lady on a silver chariot rode across the sky and shot moonbeams at the Earth with her silver bow.
But, did anyone actually believe it? The answer is most likely no. The phrase came into common usage due to its improbability. No the moon is not made of a mottled dairy product. How many cows would you need to make a cheese that big anyway? The subject of the jibe is gullibility. When people used the reference it was basically to ask, “How gullible do you think I am?” Other such phrases have been employed in language to try and convey an idea so ridiculous that it cannot possibly be believed. Someone may say, “Yeah, (in answer to someone else’s fantastic assertion) “and I have a deed for the Golden Gate Bridge right here. Wanna buy it?”
So don’t be fooled into a false belief just because someone swears that it’s the truth. Some people’s truth may be suspect, and that is why we should always investigate contentions diligently for ourselves. Even if the source has proved to be reliable in the past.