Was a little surprised I couldn't find a previous question asking about the phrase "Ain't Seen Nothing Yet". I try to decode its meaning every time I hear the song of the same name by Bachman Turner Overdrive.
If you process each word by itself it seems (to me) that the message is "You have not seen nothing; therefore, you have seen something"; but I don't think this is the intent of the phrase. Maybe I've over-analyzed this too much? The addition of "yet" even further complicates things.
Music Lyrics snipplet for the unmusical:
I met a devil woman
She took my heart away
She said, I've had it comin' to me
But I wanted it that way
I say that any love is good lovin'
So I took what I could get mmh, mmh, mmh
She looked at me with them brown eyesAnd said, You ain't seen nothin' yet
B-B-B-Baby, you just ain't seen n-n-n-nothin' yet
Here's something that you're never gonna forget
B-B-B-Baby, you just ain't seen n-n-n-nothin' yet
And you're thinkin' you ain't been around, that's right
EDIT: Definitely not looking for lyrics interpretation… this song is the only time I really hear the phrase however and it distracts me every time I hear it.
Best Answer
Aside from some very marked contexts[1], "You ain't seen nothing yet", (and the equivalent using standard verb forms "You haven't seen nothing yet"), does not mean, and never has meant, "You have seen something" in any dialect of English. This is because English is not logic, it is a human language and used by humans, and humans like to pile on the negatives when they want to express something negative.
After somebody invented a rule that said you shouldn't say "not seen nothing", generations of pedants and pedagogues have made this ridiculous claim (that the negatives cancel out) in order to provide some rationalisation for the arbitrary rule.
"You ain't see nothing yet!" unambiguously says "you haven't seen anything yet", with the implication that "what you have seen is nothing in comparison with what is to come".
[1] One can concoct an example like "You think that's got nothing? No, you watch (name some film the speaker thinks is boring) and then you'll see nothing. You ain't seen nothing yet!" where the two negatives do cancel out. But this depends both on context and on particular intonation.