To strike or hit something, often with a hollow sound, or in poker, to play poorly and lose a hand you should have won.
Likely onomatopoetic in origin, mimicking the sound of a hollow impact. The poker usage is modern slang emerging from informal card game culture, probably from the sound of chips hitting the table.
In poker, a 'donk' is actually someone who makes weird plays that don't follow strategy—and the word might come from calling them a 'donkey' for being stubborn and illogical, which got shortened over time in gaming communities.
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