means A slow-moving reptile with a bony shell enclosing its body, found on land and in water (and, in many dialects, used loosely for tortoises and terrapins too).

from From the French 'tortue,' the name for these shelled reptiles. English speakers seem to have reshaped 'tortue' into 'turtle' — likely by leaning on the familiar bird-name 'turtle' as in 'turtledove,' which is unrelated and comes from Latin 'turtur,' an imitation of the bird's cooing call. So two very different creatures collided in one word: the cooing dove and the plodding reptile, joined by nothing but the accident of sound.