To forbid, prevent, or refrain from doing something out of fear or caution (archaic); to forsake or avoid.
From Old English 'for-' (forbidding prefix) + 'fear' (to fear, dread). The verb combines a negating/preventing prefix with the emotion of fear, creating a sense of being prevented by fear from action—eventually replaced by 'forbid' and 'refrain.'
The word 'forfear' exists in Middle English texts, but it lost out to 'forbid' and 'refrain,' which were simpler and didn't require parsing the 'for-' prefix. It's a textbook example of language pruning unnecessary complexity.
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