If you have the need to nest 5 levels of for-loops, I suggest taking a step back and rethinking your approach, my friend.

Even if that other approach is just refactoring it into separate methods.

I just do i2, i3, etc

Two or three "i"s is readable, but any more and you’re counting.
I’Ve started using i, k, m, n that’s usually enough.

At this point we might as well go full Roman as you suggested. MXMCIIV to MXMCCVII as indices.

When you have multiple indices you’re also bound to have multiple cardinals those indices count up to, say foo.length and bar.length, so foo_i and bar_i are perfectly legible and self-documenting. A bit Hungarian but Hungarian is good in small amounts. Unless you’re dealing with width and height in which case it’s x and y but it’s not that width_i would be incomprehensible.