The world of down is not as expansive as the world of up. Up, as noted last week, has such appeal that people even stick it reflexively into phrases where it is not required (clean up a room).
But the flexibility of down is not to be played down, or even downplayed. Like up, it can be a noun (the third down), a verb (the sniper downed the plane), an adjective (a down expression), an adverb (Jack fell down) and a preposition (goin' down the road). Like up, down is so versatile that it can contradict itself without breaking a sweat. "Down with horses" is the rallying cry for people who despise the creatures. "I'm down with horses" is a slang way of saying you are comfortable with horses.
That leads to an old joke. How do you get down from a horse? Answer: You don't get down from a horse. You get down from a duck. Yes, it's our old friend the element of misdirection, feinting with an adverb and striking home with a noun.
In the sense of a nestling's first fuzzy feathering, down entered English in the Middle Ages from the Old Norse dun or dunn. This can be confusing, because dun the Old Norse down has nothing to do with dun the colour, even though some down is dun. (By the way, if you have done the Old Norse down, you may expect his friends to complain, because do down means overcome or beat up.) Dun, meaning dull brown, appears to have Celtic origins.
Then there is the football down. This originated in the late 1800s when a ball carrier, after being tackled by his opponent, would cry "Down!" to signal that he realized he was down and that all those heavy players could now get off him, please.
Of course, that sense of down depends on the basic sense, which is to proceed to a lower spot. This word, which in Old English was dun or dune, was shortened in the 11th century from adune (downward), which had been shortened a century earlier from of dune, which meant off the hill - in other words, down. It is no coincidence that the English use "the downs" to describe hilly terrain.
Wordsmith John Ayto notes that the French had a similar phrase, à val, which meant "to the valley" and was used as a synonym for down. From that came the French verb avaler, to go down, as in food being swallowed. Coincidentally, when describing the falling of great quantities of snow from the top of a mountain, the people of Savoy spoke in their French dialect of a lavantse. Those who spoke the Romansh language in Switzerland made an association between lavantse and avaler, and transposed the l and the v to form avalantze, which the French then borrowed as avalanche.
People mix and match letters that way all the time. In her new book What Made the Crocodile Cry?, Susie Dent writes about the origin of "nickname." In the 1300s and 1400s, the familiar name that others give you was known as an eke-name, because eke meant something added on. But people who spoke of an eke-name began thinking the "n" belonged to the noun rather than to the article, and started recording it as "a nickname." In the same way, Dent writes, "a nadder, a noumpere and a napron became an 'adder,' 'umpire' and 'apron' respectively." This is what happens when nobody is put in charge.
If being given a nickname leaves you down in the dumps, don't seek a garbage dump to sink into. The "dumps" in that expression has since the 1500s referred to a state of sadness, and may derive from the Middle Dutch domp, meaning a haze and possibly a foggy mental state. If you find a likelier explanation, feel free to write it down. Or write it up. Your choice.