This sentence "... the Mrs Grundies of the Queen's English Society, for example, protest too much" appears in the final paragraph of this piece in today's Daily Telegraph about lexical and pronunciation changes in English. The whole article is interesting and follows on from my post of yesterday about words whose pronunciation is changing.
A Mrs Grundy, according to the definition in the single-volume Oxford Dictionary of English (ODE), is "a person with very conventional standards of propriety". Incidentally, the dictionary gives the plural as Mrs Grundys. The OED definition is rather more picturesque "a personification of the tyranny of social opinion in matters of conventional propriety".
Mrs Grundy refers to a character, a farmer's wife, in the 1798 play Speed the Plough by Thomas Morton. She never actually appears in person, but is referred to constantly by the other characters, particularly by Dame Ashfield, another farmer's wife. This snippet is just a few lines into the first act:
Dame Ashfield. And I assure you, Dame Grundy's butter was quite the crack of the market.
Ashfield. Be quiet, woolye? aleways ding, dinging Dame Grundy into my ears—what will Mrs. Grundy zay? What will Mrs. Grundy think—Canst thee be quiet, let ur alone, and behave thyzel pratty?
The main dictionaries on my bookshelf all take a different approach to this word. Mrs Grundy is a headword in the ODE. Grundy is also a headword but readers are referred to the Mrs Grundy entry. Collins Dictionary does not have Mrs Grundy, but it has more or less the same definition ("a narrow-minded person who keeps critical watch on the propriety of others") at the entry Grundy. It also lists the derivatives Grundyism, Grundyist and Grundyite. Chambers Dictionary does not have Grundy or Mrs Grundy, but has Grundyism as its main entry with the definition "conventional prudery". The OED (not on my bookshelf, but on my computer!) has the headword Grundy and the derivatives Grundified and Grundyish, as well as those mentioned above in Collins.