Like so many other things in mathematics, I suspect it goes back to Euler (18th century). He invented the capital-sigma notation for sums, although I'm not sure what letters he used for his indices.

The use of x, y and z as unknown reals goes far as back as Descartes. I don't know what he used for integers though, or whether he bothered to distinguish integers from reals.

Maybe it stands for "iterator"... who knows.