There's no such thing as "zeroith" because it's called "zeroth — being numbered zero in a series"

This works for building storeys, this would work equally well for tables. The only reason this is not used often is because the series are rarely zero-based in anything that doesn't also want to equate index and offset.

You're right that first may be read as "opposite of last", that would add to the confusion, but that's just natural language not being precise enough.

Edit: spelling

Edit2: also, if you extend that logic, when you're presented with an ordinal number, you would need to first check all the options, sort them, and then apply the position you're asked, that's not really how people would expect ordinal number to be treated, not me, at the very least

I kind of brought this up in another comment, that "first" and "1st" aren't really the same thing. Which is confusing when you extend that to fourth/4th five/5th. I don't generally see someone write "zeroith", but I'll see "0th".

That's a problem when you get to the fourth.