by 1879, U.S. dialect abbreviation of you all (see you, and compare yins).
Children learn from the slaves some odd phrases ... as ... will you all do this? for, will one of you do this? ["Arthur Singleton" (Henry C. Knight), "Letters from the South and West," 1824]
We-all for "us" is attested by 1865; we-uns by 1864. Who-all attested from 1899.