Does this sentence make any sense to you as a native speaker? The one who said this actually meant to say, "People like you can never be sure about anything" (implying the opposite side is very ignorant), but this usage of "ain't" has never appeared anywhere I have seen; does it make any sense to say so?
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Ain't is a common, normal slang word in English. It does make perfect sense, but it sounds very uneducated.
I would suggest that "ain't" is a rather archaic slang word. It's something that a gangster in the 1930s would say.
In that particular sentence, this form: "You ain't sure about nuthin'" is more common.
Ain't also has a historical interest. The first ever "talking movie" (movie with sound) was The Jazz Singer starring Al Jolson. The film is indeed an ordinary silent film for 20 minutes, just like any other "normal" film of the day, and then—this was astounding to the audience of the day—Jolson says: "Wait a minute, wait a minute, you ain't heard nothin' yet."
So, "ain't" was the 8th word ever uttered in the movies!
In fact, my grandfather who was a very keen movie-goer, was at one of the first performances of this first "talkie," and indeed he literally dropped his cigarette and fell off his chair at the moment when Jolson "opened his mouth and words came out." It's a great family story.
I'm not going to (or should I say "I ain't gonna"?) comment on the validity of "ain't"; I use it myself once in a while, mostly for dramatic emphasis. To me, ain't able is what jumps out at me - no-one I know would formulate a sentence that way; it just doesn't flow naturally.
Instead of "ain't able", most people would simply say can't - "You can't be sure about anything." However, that's not necessarily an insult: it could be a general philophical statement (we often say "you" when we mean "one" or "anyone"), or it could be a statement about the situation: "Since your husband has been lying to you, you can't be sure about anything."
To make it clear that you want to insult the person you're talking to, you could use capable: "You ain't capable of being sure about anything." However, "capable" is a fairly sophisticated word, and it clashes with "ain't". So I would go straight to very clear insults: You ain't got the brains to be sure of anything.
By the way - since "ain't" became a grammatically taboo word, the number of its meanings has actually increased. Originally (and legitimately) it meant "am not"; as @Peter Shor pointed out, it grew to mean "am not / is not / are not"; he didn't mention that it also can mean "have not / has not", when "to have" is being used as an auxiliary. "I ain't got no shame / Doing what I like to do" (from Porgy and Bess); "I ain't dead, I ain't done / I ain't scared, I ain't run" (T.I., No Matter What)
Ain't is a contraction of all forms of be not in present tense (is not, are not, am not) as well as have not. It makes sense to use it here in your example, where it substitutes are not. Though it's considered vulgarism even now, I believe. See this article on ain't in The Free Dictionary
I'm not a native speaker, so feel free to critisize if I've said something wrong. :)
Ain't can be defined as : am not, are not, is not, have not, has not, and do not
After reading that, I got the idea that the speaker is criticizing "You" for being indecisive. I suppose this is close enough to the "true" meaning. I'm sure ain't is used in an understandable way. I won't go as far to say its used correctly, because I ain't an expert.