turtle: Turtle the dove [OE] and turtle the marine reptile [17] are different words. The former was borrowed from Latin turtur, which no doubt originated in imitation of the bird’s cooing. It is now encountered only in the compound turtledove, first recorded in the 13th century. Turtle the reptile is more of a mystery. It is generally assumed to be an alteration of French tortue ‘tortoise’ (source of English tortoise [15]), but since it is not known where that came from, it does not get us much further.
The expression turn turtle (which dates from the 19th century) alludes to the practice of sailors turning turtles over on to their backs, like beetles, so that they were helpless and could be easily captured. => tortoise
