(pron.) A contraction for "you all," used primarily in the southern United States. The possessive of "y'all" is "y'all's" as in:
"Do y'all have y'all's stuff packed for the trip?"
Regional note: Often the use of "y'all all" is perfectly acceptable for many Southerners:
"Y'all all have a great time tonight!" is equivalent to:
"All y'all have a great time tonight!"
Every reputable source states that "y'all" is used only as the second person plural pronoun (addressing more than one person). Quoting one source: Southerners do not, as is sometimes believed, use "you-all" or "y'all" for both singular and plural "you."
But this is not altogether true. In some cases, particularly when a heated exchange is taking place in a trailer community between two partners or individuals, "y'all" is perfectly normal for use as the second person singular pronoun:
"I've HAD it with yore horse shit, Jesse Sue! Y'all can go FUCK OFF!!"
NOTE: This is useful because the person addressing Jesse Sue can imply both Jesse Sue (by herself) AND her biological family members in the same statement.
"Did y'all fuck Jesse Sue last night?"
"Y'all's grandma is a whore!"
"Y'all all git y'all's ass outta here!"
"Y'all come again."