Yes, down is playable
...and is worth 9 points.
Found in the following dictionaries
Enable (Words with friends)
SOWPODS (Collins Scrabble Words)
OTCWL2014 (NASPA Word List 2014)
TWL06 (NASPA List 2006)
"Down" Definition
noun
A downward movement; descent.
A soft, silky, or feathery substance, such as the first growth of a human beard.
An expanse of rolling, grassy, treeless upland used for grazing. Often used in the plural.
Any of several breeds of sheep having short wool, originally bred in the Downs of southern England.
Botany A covering of soft, short hairs, as on some leaves or fruit.
adjective
Baseball Retired; out: two down in the last of the ninth.
Completed; done: three down, two to go.
Football Not in play: The ball is down on the 50-yard line.
Football Not permitted to advance further in the play because forward progress has stopped, especially by being tackled. Used of a ball carrier.
Learned or known perfectly: had the algebra problems down.
idiom
Down on Informal Hostile or negative toward; ill-disposed to: was down on jogging after his injury.
verb-transitive
Football To put (the ball) out of play by touching it to the ground.
To bring, put, strike, or throw down: downed his opponent in the first round.
To swallow hastily; gulp: downed the glass of water.
verb-intransitive
To go or come down; descend.
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