also: もうせつ
noun
A formal or literary term for a baseless claim, erroneous argument, or false rumor. Often used in critical or academic contexts.
その説は妄説に過ぎないと学者たちは一蹴した。
Scholars dismissed the theory as nothing more than a fallacy.
新聞に載った話は全くの妄説だった。
The story in the newspaper was a complete false report.
Compound of 妄 (もう or ぼう, 'reckless, deluded') and 説 (せつ, 'theory, explanation'). The reading ぼうせつ is the more common Sino-Japanese reading; もうせつ is an alternate reading.