folk etymology; popular etymology
A linguistic term for a commonly believed but historically incorrect explanation of a word's origin, often based on superficial resemblance to other words.
Explaining the origin of まな板 as 'true fish board' is an example of folk etymology.
この説は民間語源に過ぎず、言語学的な根拠はない。
This theory is nothing more than folk etymology and has no linguistic basis.