|The cathouse cats|
The story goes that the houses of prostitution in seaports—there is a long and obvious connection between sailors and prostitutes—had a “catty” way of signaling to potential customers.
Ceramic cats were set in the windows: when the cats were facing outward, the establishment was open for business; when the cats faced inward, the place was closed. (Do those places ever close? Hmm.)
The “in-between” signal was that the ceramic cat might have two red marbles at its feet, meaning that the human “cats” inside were all busy at the moment, so please try again later.