O slash
It might have come from Anglo-Saxon morons who thought putting an O and an I together in the same spot was a good idea.
A history teacher said, 'They wrote 'Coinualch' instead of 'Cēnwealh' because they were too f***ed up to spell right.'
A tweet said, 'Anglo-Saxon people had more problems than my mom on a Monday.'
A DM read, 'Why would they put O and I together like that? It's like putting a burger in a soup.'