fathead

"Fathead" is most often an old-fashioned and somewhat harsh term for a person seen as foolish, stupid, or lacking good sense. In this use, it does not refer to body weight. Instead, it paints a picture of someone with a head that is metaphorically thick or dense, someone who does not think clearly or refuses to listen.

This insulting sense has been around since the 19th century and was especially at home in sharp, comic, or scolding speech. It often suggests not just lack of intelligence, but also stubbornness: the kind of person who ignores advice, misses the obvious, or keeps repeating the same bad idea. Because of that tone, it can sound funny or colorful in old dialogue, but in modern use it can still come across as rude and dismissive.

You are more likely to meet this sense in older novels, films, or playful arguments than in formal conversation. Someone might say, "Don't be such a fathead," when a person is being obviously foolish. Even then, the word has a dated feel, almost like something said by an annoyed uncle, a comic character, or a voice from another era.

There is also a separate literal use of "fathead" in the names of certain fish. The best-known example is the fathead minnow, a small freshwater fish found in North America and often mentioned in fishing, biology, and environmental writing. In that context, the word is neutral and has nothing to do with insult.

So the word has two main paths: a dated insult for a foolish or thickheaded person, and a literal name used for some fish. The insulting sense is the one most people notice first, but the fish sense is also established and important if you want the full picture.