The story
- Proto-Indo-European*yúHs
- Proto-Indo-European*uswé
- Proto-Indo-European*uswís
- Proto-Germanic*izwiz
- Proto-West Germanic*iwwiz
- Old Englishēow
- Middle Englishyow
- Englishyou
- Proto-Indo-European*h₂el-der.?
- Proto-Indo-European*h₂elnós
- Proto-Germanic*allaz
- Proto-West Germanic*all
- Old Englisheall
- Middle Englishall
- Englishall
- English todayy'all
Contraction of you all. Attested since at least 1631. Compare Dutch jullie (originally jij lui (literally “you people”)) for a similar development of a new plural pronoun out of Proto-Germanic *jīz that originally was already plural.
Etymology from Wiktionary, CC BY-SA 4.0.