Duck - repost

I would really love to know the origin of the word "duck" to mean that you should move downwards. Of all the things that ducks do, actually ducking probably isn't one of the more common ones. I mean they very commonly fly, hell they even fly in groups, thus flocking. and flock sounds more like duck. Ducking isn't really something you see real ducks doing. If some shit is in their way they usually fly over. The word makes no sense. I'd be like me yelling at someone who stopped breathing "FISH! FISH DAMMIT! COME ON FISH!".