The Oxford English Dictionary (OED) is usually academics’ first go-to for all things linguistic, and it cites the first occurrence of “you all” in print in 1824, from Arthur Singleton’s Letters from the South and West:“Children learn from the slaves some odd phrases;..as..will you all do this?”This example is important for understanding not only the term’s origins but also the difficulties surrounding it.First, does this count as “y’all”? It doesn’t contain the contracted “y’all” but rather the spelled out “you all,” a phrase that occurs all the way back in Middle English texts; however, given that Singleton is recording the instance as an “odd phrase,” has he attempted to correct it by spelling out the contraction? Or has he recorded the phrase as it was spoken, so that this wouldn’t really be considered an instance of “y’all” at all?

Source: The secret history of “Y’all”: The murky origins of a legendary Southern slang word – Salon.com