The difference between a turtle and a tortoise can be confusing: Technically, all turtles, tortoises and terrapins belong ARE turtles and belong to the turtle order Testudines, which comes from the Latin word testa, meaning “shell”
But for the most part, people use the word “tortoise” to refer to completely land-dwelling turtle species, while the generic word “turtle” refers to turtle species that are at least partly aquatic.
Unfortunately the etymology of these words only makes matters more confusing.
Before the 14th century in Old English, “turtle” was a word for what we now call a turtledove. The word “turtle” was meant to sound like a turtledove’s cooing.
Only in the 1600s did the word turtle become a word for the reptiles, when the French tortue was misunderstood as sounding like the English “turtle.”
Before that, the English word for the reptiles was “tortoise,” and aquatic turtles were known as “marine tortoises.”
So “turtle” is an older English word than “tortoise,” but “tortoise” is an older word for a turtle than “turtle.”
The word “tortoise” comes from the Middle English word tortuse. The spelling of it was influenced by the ending of “porpoise,” which literally means “pig-fish.”
The Middle English tortuse either comes from the Latin word “tortus,” meaning “twisted” after the shape of their feet, or it could be from the Greek Tartaros, which is the name of the sunless abyss below Hades. The Tartaros theory is supported by the fact that turtle species were thought to basically look like little monsters.
In fact the French word tortue, was also often associated with diabolical beasts.
Useless Etymology 