To drink something quickly, to knock something over or flatten it, or to hit an opponent in sports.
From 'down' (Old English 'dun'), which originally meant 'hill' but evolved to mean 'toward the ground.' Adding '-ed' makes it past tense, and the verb developed multiple meanings based on the direction of movement.
The same word for drinking a glass and hitting someone in football shows how English loves to reuse 'down' for anything involving quick downward motion—it's efficient but sometimes hilarious.
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