"Faux pas" literally means "false step". But "pas" in French also means "not". There is a city in northern France called "Pas de Calais", which roughly means the "steps" or entrance of Calais (there's a strait, and the Allies successfully fooled the Germans into thinking it would be the D-Day landing site).

For a time, while not knowing that "pas" also meant "step" I wondered why in the world anyone would name a city "not from Calais". :-)