One must remember that “turtle” applies, scientifically speaking, to the marine member of the family, and that “tortoise” should properly be used to describe the land or fresh-water member.
The sea beastie is the one most highly prized for food, nowadays chiefly appearing on the menu as “turtle soup.”
Our English-speaking ancestors, however, called both the land and sea species “tortoises” until the middle of the seventeenth century, for it was not until then that they began to realize that the huge marine creature, unknown to them before the travels of Columbus, Cabot, and other explorers, was not just an overgrown land specimen.
Sailors started to speak of this animal as a “turtle,” probably thinking that they were giving it the French name, tortue, though in reality they were giving to this quadruped a name that had previously been applied only to the bird which we now call “turtle dove.”
Also son.n. time in the seventeenth century, sailors found that these sea monsters were edible, if one could first capture them. Caribbean natives, they observed, waited until one of the reptiles came ashore to lay her eggs, then, seizing her by one of her flippers, turned her over on her back.
Thus careened, she was absolutely helpless. This the sailors called “turning the turtle.” Later, because a ship that had been capsized bore a fancied resemblance to an overturned turtle, sailors called such capsizing, “turning the turtle.”
Ashore, that saying was adopted for anything that was upside down.