Down is the negative direction along the z-axis. Up and down are the only two well-defined directions in the presence of gravity. Earlier this was adown, meaning "from the hill".
Down is the fine undercoat of birds under the feathers, and is often used as insulation in clothing. Mammals have an undercoat called downhair.
Down, used in American football, is a stage of play; four downs are allowed in a "possession"; three downs are allowed in Canadian football. The reference is to the ball or ball carrier being brought to a halt, often by being knocked down.
Down is a colloquial synonym for depressed or blue.
A computer (or other) system can go down when it essentially loses power or crashes.