Prevented something from succeeding or blocked someone's plans; stopped or frustrated an attempt to do something.
From Old Norse 'þvert' meaning across or transverse, ultimately related to 'thwart' as a nautical term for a seat running across a boat. The meaning evolved from the spatial concept of something going across or blocking a path to the metaphorical sense of preventing success.
The nautical origin of 'thwart'—a seat that runs across a boat—is wonderfully preserved in the word's meaning; just as a thwart seat sits across the boat's direction, something thwarted blocks or cuts across your intended path and prevents forward movement.
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