Knocking something or someone down, or quickly consuming food or drink.
From 'down' (Old English dūn, meaning 'from the hill'). The verb form developed in Middle English, originally meaning to force downward. The sense of 'consuming quickly' arose in American slang by the 1900s.
The word 'downing' is a perfect example of how the same verb can mean completely opposite types of actions—you can 'down' an enemy in combat or 'down' a glass of water in celebration. Both involve speed and force, which explains why one word works for both.
Complete word intelligence in one call. Free tier — 50 lookups/day.