The word duck comes from Old English *dūce "diver", a derivative of the verb *dūcan "to duck, bend down low as if to get under something, or dive", because of the way many species in the dabbling duck group feed by upending; compare with Dutch duiken and German tauchen "to dive".
This word replaced Old English ened/ænid "duck", possibly to avoid confusion with other Old English words, like ende "end" with similar forms. Other Germanic languages still have similar words for "duck", for example, Dutch eend "duck" and German Ente "duck". The word ened/ænid was inherited from Proto-Indo-European; compare: Latin anas "duck", Lithuanian ántis "duck", Ancient Greek nēssa/nētta (νῆσσα, νῆττα) "duck", and Sanskrit ātí "water bird", among others.
Some people use "duck" specifically for adult females and "drake" for adult males, for the species described here; others use "hen" and "drake", respectively.
A duckling is a young duck in downy plumage or baby duck; but in the food trade young adult ducks ready for roasting are sometimes labelled "duckling"
And because no duck post is complete without a picture: