It is difficult to understand an English sentence.

1, ain not is the abbreviation of "am not, are not, is not, have not". It was originally said by a group of illiterate rude people (but not indigenous people) in the southern United States, but now it tends to be universal and has been included in various dictionaries. It started with the abbreviation of am not. At first, it was written as am n' t, but later, due to the problem of pronunciation linking (omitting the previous consonant m and only pronouncing the last consonant n), it became ain 't. Obviously, it consists of a be verb am and a negative word not, so in principle it only applies to any abbreviation of am not.

In Britain, ain generally only uses the working class, such as people who speak London dialect, which is usually considered inappropriate. The middle class and the upper class, as well as Britain in the19th century, are willing to use it in familiar speeches and the education of the upper and middle classes.

It rarely appears in formal writing, and is usually used in more informal written vocabulary, such as the lyrics of pop songs.

What you said is very reasonable. Nobody itself is "nobody", and then it is used in negative sentences, which is really contradictory. However, we should pay special attention to the fact that the format of English poetry is different from normal communication language, and the grammatical phenomenon of poetry is not standardized. It only needs reasonable meaning and rhyme, and is generally accepted by people. The second half is like this if it is said in a complete grammatical structure. There is no one to keep us down.

Lyrics are lyrics and cannot be changed. If they are changed, the author's intention will also change.

It seems that you are an English lover. I wish you progress in your study!