I generally hear y'alls's used as the possessive form, but I have also heard yourn. Since y'all is a colloquial pronoun, its possessive form is basically liberated from prescriptive linguistics which would probably say that y'all is a contraction for you all and therefore must be possessivized as of all of you.
Do you know if there is a "standard" way to make y'all possessive, insofar as y'all usage can be considered standard?
Best Answer
It depends on whether you are indicating individual or group possession.
Your is used because each person has a coat.
Y'all's is used because the house is theirs collectively. I never use y'alls's because y'all is already plural and doesn't need the s before the apostrophe.
For what it's worth, I'm from the Alabama and use both y'all and y'all's regularly.