Trembling or shaking, often from fear, cold, or physical vibration.
From Old English 'cwacian' (to shake/tremble), which may be imitative in origin—the word sounds like shaking. Related to German 'quacken' and may relate to the concept of quivering.
The Quakers got their nickname partly because early meetings involved people trembling or 'quaking' as they felt religious emotion—and the 'qua' sound in 'quaking' almost sounds like teeth chattering!
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