Good memory, it’s in Lysistrata, but it’s a joke related to how the women withheld sex to stop a war. Yow, this PD translation is ye olde archaic English translation —
[snip]
HERALD My scrimp-brained lad, I’m a herald, as ye see, who hae come frae Sparta Anent a Peace.
MAGISTRATE Then why do you hide that lance That sticks out under your arms?
HERALD. I’ve brought no lance.
MAGISTRATE Then why do you turn aside and hold your cloak So far out from your body? Is your groin swollen With stress of travelling?
[/snip]
Yes I remember it was from Lysistrata but maybe a different scene.
The Athenian sees the Spartan and asks what is that under your cloak. Are you a man or Priapus?
The Spartan herald says: no you fool, it is a Spartan message rod
He is referring to the way the Spartans would send encoded messages using rods wrapped with velum. there would be holes in the sheep skin which could only be read with a corresponding rod. I dont recall exactly how it worked but have read that it was a sophisticated code that would probably work even today.