Discover English, an EFL textbook for children published by Pearson, mentions girls dancing to Will Smith's Switch in Chapter 1. I was using the chapter to teach English via Skype to an eleven-year-old girl, and wondered if I could incorporate some of the lyrics of the song in the lesson. I couldn't, because I didn't understand them myself, but I noted with interest Smith's use of y'all.
By coincidence I had just read the short chapter, or interlude, on y'all in David Crystal's The Stories of English. He says that y'all originated in the southern states of the USA at the end of the 19th century. It was first mostly used by African Americans, but its usage among all social groups of all American regions soon spread. Crystal notes that y'all is usually used when addressing a group of people, but he has also heard it used to address two people, and to address a single person.
Standard English has one word for you, although it used to have an informal singular form thou (thou was the subject, and thee was used when the object of the verb). The current word you was once only the object form; the subject form was ye. Some dialects still differentiate between you in the singular and you in the plural. My Irish in-laws frequently say ye to address a group and my own dialect (Liverpool) has youse. A friend of mine who taught in a school in Liverpool about twenty years ago said one day to a boy in her class 'Stand up, Hughes' and every child in the class stood up!
Like y'all, youse sometimes refers to just one person. By coincidence, I spotted a written example of this in yesterday's Daily Mail. The sports correspondent (or maybe he's the showbiz correspondent) had criticised Wayne Rooney in an article a few months ago, and now he found himself a few yards away from the footballer at Dubai Airport. Mrs Rooney -- Coleen, who is a Liverpudlian like her husband -- came up to the (one) correspondent and said "What gives youse the right to criticise us?"