Wikipedia tells us:

"Mrs Grundy is the personification of the tyranny of conventional propriety (from Thomas Morton's play Speed the Plough, which appeared in 1798).
(By contemporary rules of punctuation of 1798, still prevailing in North America today, she is Mrs. Grundy.)
Peter Fryer's book Mrs. Grundy: Studies in English Prudery concerns prudish behaviour, such as the use of euphemisms for underwear.
By the mid-nineteenth century, Mrs. Grundy was so well established in the public imagination as a canonical character that Samuel Butler, in his popular novel Erewhon, could refer to her in anagram (as the goddess Ydgrun).
Robert A. Heinlein also mentions her, for example, in his novel The Number of the Beast."

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