What do you mean, you don't have hair on your mouth and you don't do things neatly?

As the saying goes, if you have no hair on your mouth, you are not practical. There are two meanings.

The first meaning, "hairless mouth" refers to young people. It means that young people may make more mistakes because they are inexperienced; Older people may be more sophisticated. That is, we usually say "you are too young".

The second meaning, "hairless mouth" refers to women. It means that women are less reliable than men. This is obviously discrimination against women. As we sometimes say, "hair is long and knowledge is short."

So "no hair on your mouth" is just a metaphor for young people or women. There is no necessary connection between being firm and having hair on your mouth. Just as the length of knowledge is not necessarily related to the length of hair. These are metaphors, and they are discriminatory metaphors.