Cretin is a very rude word for a person you think is extremely stupid. Do not use it. It is meant to hurt, and in English it is strongly offensive. Though actually, it did not start as a schoolyard insult. It came into English from French crétin, and that likely came from an Alpine way of saying the word for “Christian” — in an old sense of “a poor fellow”, anyone in the Christian world. In the mountains it was also used for people with a serious illness: when a child did not get enough of a kind of salt the body needs, the body and mind might not grow in the usual way. Later, speakers pulled the word away from that medical use and turned it into an insult for anyone they wanted to call stupid. That history does not make the insult kind. A student only needs this word to recognise it in a text and to stay away from it. It shares an old root with Christian, but the two words do not mean the same thing now.