Learn English – “Leave nothing (left) behind”

To understand what this simple phrase means first think of the literal meaning, that the person was abandoned because it was assumed that they were already dead, mortally wounded or that it was inevitable that they die.

But there is more to the phrase than that. In common usage, this phrase is often part of a narrative where a person was expected to die, yet against the odds they survived. Some examples from COCA:

Left for dead by a busy road, Susan managed to get a ride to a police station where she insisted she be brought to the Nairobi Women's Hospital.

Pueschel, just 11-years-old, was beaten, stabbed and left for dead, but he lived to testify against Reginald and Gerry Mahaffe.

... a young man in Los Angeles was robbed, shot, left for dead, but he lived because a bullet was stopped by a radio in his pocket.

The common theme in these stories is an act of cruelty towards an innocent person who survives either by a stroke of luck or a miracle. A proper reading of the title Left for Dead should include cruelty, abandonment but also a sense of hope.

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.