fornix
For´nix
Noun 1. fornix - generally any arch shaped structure (but often it refers to the arched roof of an anatomical space)
body part - any part of an organism such as an organ or extremity
2. fornix - an arched bundle of white fibers at the base of the brain by which the hippocampus of each hemisphere projects to the contralateral hippocampus and to the thalamus and mamillary bodies

The walls of Rome had many "fornix"s, and I have read that
they were commonly used as places for Roman prostitutes to
solicit passersby, and thus source of word 'fornicate'.

Fornix also meant "brothel, bagnio, stew" in Latin. Fornicor means "to frequent whores, to fornicate". I've seen it suggested that brothels were in cheaper rent, underground, vaulted areas. There's a whole street full of brothels in Pompey and the graffiti on the walls is filthy (in Latin and Oscan).

This reminds me of one of my favorite terms for this activity: to swive; cognate with our modern word swivel. You see it used back in Early Modern English or Late Middle English.