The only free and comprehensive online etymological dictionary of the Spanish language
ello n. (Pronoun) "it"
From Latin illud "that one," from Old Latin ollum. From Proto-Italic *olno- "that." See él.
Indo-European
Italic
Oscan úlleís "of that," Umbrian ullu "toward that"
Celtic
Gaulish ollo- "beyond," Old Irish ol 'id.,' Middle Welsh ol "according to"
Balto-Slavic
Old Church Slavonic lani "last year," Czech loni 'id.'