I'd never heard this expression before, but we once had a porch cat. Some inane and cruel person dropped of a young cat in the countryside near our home, and she eventually came looking around our yard for food and shelter. I have a self-imposed I'm-not-going-to-turn-into-the-weird-cat-lady two-cat limit, and at the time our household was already at the limit, so I couldn't adopt her ... BUT ... we fed her and tended to her and looked for a home for her. We didn't let her inside, so she would meet us on the front porch, and so we started to call her our porch kitty. Then, of course, we named her Porsche.

Happily, Porsche ended up as the pampered pet in a one-cat, no-kids household.