I learned recently that the words ”tortoise” and “turtle” (and their homologues in French, Spanish, Italian, etc.) appear to be derived ultimately from Tartarus, the lowest depths of the classical underworld, from some medieval association of these unlovely, innocuous creatures with demons and demonic powers. Some dictionaries offer a more prosaic alternative, from Latin tortus, twisted (from the shape of the legs). I don’t know about Germanic tongues.
The classical Latin name for a tortoise, testudo, refers to the creatures’ hard carapace. Roman infantry advancing to attack, would sometimes form a sort of canopy of interlocking shields, to protect themselves from arrows and javelins; this was also called testudo.
Tartarus is a reminder of how steeped in superstition our medieval ancestors were (not that we’re much better nowadays: only the superstitions are different ones).