How does one form the possessive of y’all: with ’s, or with an s and no apostrophe? I sort of want the answer to be y’all’s, because I like the way it looks, and I like that it sometimes makes my students mad when I write it that way on the board (without telling them that it’s really wrong), and because it’s one word that illustrates both uses of the apostrophe (to form both contractions and possessives).
Personal or definite pronouns--that is, pronouns that refer to something definite and that have a clear antecedent--do not have apostrophes in their possessive forms. Examples of these pronouns are he, she, you, and it. The possessive form of these pronouns: his, her, your, and its (not he’s, and of course not it’s, which is not a possessive but is a contraction for it is).
Some pronouns, called indefinite pronouns, do use an apostrophe to form the possessive. Example: anybody, a pronoun that, unlike those above, does not refer to anything specific and has no real antecedent. (“Anybody can grow up to be president.”) The possessive of anybody is anybody’s, with an apostrophe. Other examples of indefinite pronouns: everybody, someone, and nobody.
But y'all is not an apostrophe-taking indefinite pronoun; it is a personal pronoun, and hence the possessive does not get an apostrophe: y’alls.
Another good question: Why am I bothering with this? Because I came across the following today in Wikipedia’s entry on y’all: “There is some debate on the spelling of the possessive form of y’all. Some will spell it ‘y’all’s’ while others will spell it ‘y’alls.’ As there does not seem to be an official answer, it is a matter of personal preference.”
So here you have it, Wikipedia, the official answer: y’alls.
(And yes, I do claim to be official when it comes to y’all.)