I was wondering if the standard notation of denoting a complex variable by "$s$" had an interesting origin, or if it dates back to Riemann or Weierstrass. Almost every book in analytic number theory seems to uses that alphabet with

$ s = \sigma + i t$

denoting its real and imaginary parts.

I shall be happy if anyone could enlighten me about it. I tried searching MO for relevant questions but couldn't find it.