All Y'all's Meaning

(idiomatic, chiefly Southern US, colloquial) Possessive of all y'all (standing in for all your).

Example: 1946, Mary King O'Donnell, Those other people, page 24:
  "How all y'all's folks, Mis' Clark? How yo maw and paw? How Dan? ...
2008, Matt de la Peña, Mexican whiteboy, page 97:
  And you know what's on the tip of all y'all's tongues? Each and every one of y'all?"