But that’s the only way to write it if you’re a mathematician.
Mathematicians aren’t locked into one type of notation. But that’s missing the point. I was pointing out that any explanation that relies on the way the OP wrote the joke is most likely wrong, since that’s not the way the joke was written originally (in the OP’s linked-to site).
Negative i
Negative as in “bad”. A bad eye. The evil eye.