The story
- Proto-Indo-European*h₃en(H)-
- Proto-Indo-European*h₃én(H)os
- Proto-Italic*onos
- Latinonu
- Englishonus
- Proto-Indo-European*h₂eyḱ-
- Proto-Indo-European*h₂eh₂óyḱe
- Proto-Germanic*aiganą
- Proto-West Germanic*aigan
- Old Englishāgan
- Middle Englishowen
- Englishowe
- English todayoweness
An eggcorn of onus, as owe (“to be under obligation”, verb) + -ness (abstract-noun suffix), interpretable as "the state of being under obligation." The eggcorn may have been produced due to the frequency of the collocation "the onus [is/was] on [someone] to," whereas onus is very rarely used in popular speech without this collocation.
Etymology from Wiktionary, CC BY-SA 4.0.