This kind of preposterous claim is completely groundless.
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history
Echoes language from 《史记·孟子荀卿列传》, which describes wild speech as 'vast and unfounded' (闳大不经). 经 here means 'standard / norm', so 不经 = 'departing from the norm'.
usage
Stronger than 荒唐 (huāngtáng, absurd) — 不经 emphasizes that the claim violates accepted reasoning, not just that it's silly. Use for theories, rumors, or stories presented as fact.
