Every writer who wants to unmistakably make a character southern uses it...and at least half of them get it wrong. It's y'all, and it's been offending southern readers since the first author used it the wrong way. Before you attempt to insert it into one of your books, make sure you understand proper use of the word y'all...or you'll end up hearing about it from me.
We're in the South, Y'all
I touched on the topic of y'all briefly in my apostrophes post, but it's so commonly mis-used it deserves to have its very own spotlight. First, let's all get clear on the meaning of y'all right here and now.
If you read my post, you know how to use apostrophes, and you know that y'all is really you all. The apostrophe takes the place of the o and the u to create this contraction. So every time you see y'all, think you all. The phrase means the same thing as all of you or (as Yankees might say) you guys.
It's a plural word, and that's the most important thing you've got to remember about y'all. It only addresses more than one, because Robby by himself cannot be an all. Robby and Johnny can be an all because together they're two guys. For example, it's totally appropriate for me to say Robby and Johnny, y'all come in here and get your lunch. But I would never say Robby, y'all come in here and get your lunch.
So many, many authors get that wrong. The word means you all, and actual southerners only use it when they're addressing more than one. Replace y'all with a phrase that means the same thing, and you'll see what I mean:
Robby, all of you come in here and get your lunch.
Robby, who's out there alone, is going to think the speaker has gone crazy. And readers who see the error are going to roll their eyes...maybe away from your book, and onto the next. Y'all means the same as all of you and it's always plural.
But I know why people get confused...because y'all's is also considered to be a usable word in the south.
It looks like the worst of grammar, but honestly it's not as incorrect as it appears. Y'all's is a double contraction that's commonly spoken south of the Mason Dixon line, and if you're writing true southern dialect it's bound to crop up. Do you know exactly what it means?
Just extend it to find out. We know that y'all is you all, so what does it mean when you add the new apostrophe and and s to the word? The same thing it always means -- it's possessive.
Y'all's refers to something that belongs to you all. For example, I could have said Robby and Johnny, y'all's lunch is getting cold instead, and it would still be correct. If something belongs to more than one and I'm addressing those owners directly, the thing becomes y'all's.
Some sources might argue with me on that one; they say the possessive form of the word is more properly spelled y'alls. Some grammarians compare y'all's to its, which is a possessive form that does not have an apostrophe. It's one of those confusing rules of English. However, according to the urban dictionary (the authority on such things), the word is properly spelled out as y'all's. I find this to be the less confusing spelling, at any rate, because you can look at it and figure out exactly what you're doing. Start deleting apostrophes arbitrarily, and you're bound to get yourself screwed up.