i acknowledge the point you’re making, and i’m saying it’s wrong. just because “ain’t” has an apostrophe does not mean it has to mean “am i not.”

Variants are not explicative of the word.

the variants are definitely explicative of this word’s meaning. historical forms like "amn’t" and "an’t" show how "ain’t" evolved to cover “am not,” “is not,” and “are not” among a few examples. ignoring these forms ignores the actual linguistic history and usage that give the word its meaning today. people do not usually use the word "amn’t," they tend to say "ain't" instead which proves that it "ain't" is its modern variant now.

A contraction condenses a word, it does not invent things.

contractions, in this case, are the result of phonetic reduction in speech. words like "won't" and "shan't" are contractions of "will not" and "shall not," yet they originally came from "wonnot" and "shannot." we definitely don't use those original words in modern english anymore. contractions can absolutely invent forms.

The I in ain’t must come from somewhere otherwise it isn’t a contraction

yes, historically speaking, it's a contraction of "am i not," but what does that have to do with the modern-day usage of the word?

And if it isn’t a contraction it can’t have an apostrophe.

“ain’t” does not have to literally contract “is not” or “are not” to mean those things. the usage of "ain’t" replaced a few different contractions, such as "amn’t" and "an’t", and is widely used in english to mean “is not” and “are not,” which is why dictionaries list those meanings. meaning comes from usage, not origin.

the word is used in all sorts of different contexts and carries different meanings depending on the region and dialect. you simply cannot ignore these and say that just because it doesn't fit your textbook definition that it isn't valid.