Ordinal vs. cardinal. It's "first" not "onest", right? Even the ancient proto-Germanic speakers could tell there's a difference. (In fact, it's basically a contraction of "foremost", and has nothing to do with numbers; their weak numeracy was an advantage on this topic)

If we weren't implicitly choosing 1-indexing it would be 1nd for "second" (and still not "onend" or something). That breaks down once you get to third and fourth, though.