Tournedos is one of the many French food names and cooking terms that have been carried over into English directly, such as foie gras, bouquet garni, génoise, etc. The exhaustive Larousse Gastronomique says you can consider the tournedos to be a small, round slice taken from near the end of the tenderloin, which is normally sautéed or grilled. In common usage, any part of the tenderloin is apt to be called tournedos on a restaurant menu. right
The French Dictionary of the Academy of Gastronomes gives one account for the origin of the name, which first made its appearance in the mid 1800s. "In the last century, the stalls backing onto (tournant les dos) the central alleys of the fresh fish pavilion, in the Paris Halles [markets], were assigned fish of doubtful freshness. By analogy, the name tournedos was given to pieces of filet of beef that were kept for a few days in storage. An indiscretion is said to have led to the word’s appearing on a restaurant menu one day; the public, not knowing its origin, adopted it."
Larousse says another possibility is that the great opera composer Rossini placed an extravagant order for filet mignon with foie gras and truffles in a restaurant that surprised the headwaiter so much, he had the dish served behind the backs (dan les dos) of the other customers.
You can probably increase your tips by coming up with a much more creative story for your customers about the origin of the word, but at least now you have some context for your tale.