The phrase one might hear a pin drop, and its variants, mean: the silence and sense of expectation are intense.

A different phrase, one can hear a pin drop, and its variants, mean: one has a keen sense of hearing.

 

ONE MIGHT HEAR A PIN DROP

 

The earliest occurrences that I have found of the phrase one might hear a pin drop and variants are as follows, in chronological order:

1-: From a letter, dated St. Petersburg, 30^th July 1739, published in Letters from a lady, who resided some years in Russia, to her friend in England. With historical notes (London: Printed for J. Dodsley, 1777) [page 191]:

The marquis Botta, the emperor’s minister, took the title of ambassador, for three days only, in order to demand the princess in form, in his master’s name, for the prince of Brunswick, his nephew. […] On Monday he had his audience, to demand the princess; her majesty stood under a canopy, on a throne raised twelve steps, at the end of the great hall […]. The ambassador had a great train, and after he had made his bows, mounted the steps of the throne, and put on his hat, while he made his speech, but took it off, when he delivered the letter. The great chancellor answered the speech, and then the ambassador withdrew, and her majesty remained in the same place, and the duke of Wolfenbuttle’s minister came and made a speech, and delivered a letter, but at the foot of the throne, and uncovered. During all this there was so profound a silence, that you might have heard a pin drop, except the speeches, which, added to the richness of her majesty’s dress, the dignity of her person, and the finery of all the company, made the ceremony very solemn and magnificent.

2-: From a letter, dated London, 11^th February 1751, published in Letters written by the Late Right Honourable Philip Dormer Stanhope, Earl of Chesterfield, to his son, Philip Stanhope, Esq; Late Envoy Extraordinary at the Court of Dresden (London: Printed for J. Dodsley, 1774), by the British statesman, diplomat and wit Philip Dormer Stanhope (1694-1773), 4^th Earl of Chesterfield [Vol. 3, page 125]:

Know that no man can make a figure in this country, but by parliament. Your fate depends upon your success there as a speaker; and, take my word for it, that success turns much more upon Manner than Matter. Mr. Pitt ¹, and Mr. Murray the solicitor-general ², uncle to Lord Stormont, are, beyond comparison, the best speakers; why? Only because they are the best orators. They alone can inflame or quiet the House; they alone are so attended to, in that numerous and noisy assembly, that you might hear a pin fall while either of them is speaking.

¹ William Pitt (1708-1778), 1^st Earl of Chatham, was a British statesman, Prime Minister of Great Britain from 1766 to 1768.
² William Murray (1705-1793), 1^st Earl of Mansfield, was a British barrister, judge and politician.

3-: From a letter, dated 26^th November 1754, from Henry Fox ³ to William Cavendish ⁴, published in the Appendix to Memoirs from 1754 to 1758 by James Earl Waldegrave K G ⁵ (London: John Murray, 1821) [page 148]—“the house” refers to the House of Commons:

I did not come in till the close of the finest speech that Pitt ever spoke, and perhaps the most remarkable […].
[…] Displeased, as well as pleased, allow it to be the finest speech that was ever made; and it was observed, that by his first two periods, he brought the house to a silence and attention, that you might have heard a pin drop.

³ Henry Fox (1705-1774), 1^st Baron Holland, was a British politician.
⁴ William Cavendish (1720-1764), Marquess of Hartington, 4^th Duke of Devonshire, was a British politician.
⁵ James Waldegrave (1715-1763), 2^nd Earl Waldegrave, was a British politician.

4-: From Memoirs of Mr. Deputy Numscull. Illustrated with an elegant Copperplate, from an original Design, published in The Town and Country Magazine; Or, Universal Repository of Knowledge, Instruction, and Entertainment (London, England) of July 1785 [Vol. 17, No. 183, page 376, column 1]—“Chatham” designates William Pitt (1708-1778):

MR. Deputy Numscull is of a very ancient and numerous family, though we do not find any noble blood ever ran directly in his progenitors veins. His father was of that useful body of men, vulgarly called coblers, and was a near kin to brother Crispin of Cripplegate: he was likewise a dabbler in politics, and every night was lecturer-general at the two-penny club in Grub-street. He always commanded the most attentive silence, as soon as he took up the paper, snuft the candle, and put on his spectacles: like another Chatham, in the House you might hear a pin fall, when he began to give the State of Europe, which Mr. Numscull sen. did with a Stentorian voice, and great accuracy—save that now and then he clapt Marseilles in America; and, by a kind of Irish instinct, slapt Bombay near Corke; but these were trifling mistakes, the intelligence was greedily swallowed, washed down with Calvert’s entire butt-beer, and completely illustrated with the fumes of Holden’s best Virginia.

5-: From The History of Sandford and Merton, A Work Intended for the Use of Children (Belfast: Printed by Daniel Blow, 1787), by the British author Thomas Day (1748-1789) [Vol. 2, page 59]—“that good book” is the gospel of Jesus Christ:

Indeed I have heard of that good book; Mr. Barlow has often read part of it to me, and promised I should read it myself. That is the book they read at church; I have often heard Mr. Barlow read it to the people; and he always reads it so well and so affectingly, that every body listens, and you may hear even a pin drop upon the pavement.

6-: From A Serio-Comic and Admonitory Epistle, Addressed to a Certain Priest (London: [s.n.], [1793]), by Anthony Pasquin, pen name of the English satirist and miscellaneous writer John Williams (1761-1818) [page 18]:

Obeisance made; with due respect and fear
They sit; the dictates of their lord to hear.
[…]
Such and so still the gaping audience sat,
To hear the priest begin the great debate.
Fear and attention occupi’d them all;—
Did a pin drop; you’d plainly hear it fall.

 

ONE CAN HEAR A PIN DROP

 

The first two occurrences that I have found of the phrase one can hear a pin drop and variants are as follows:

1-: From Les Amusemens de Spa: Or The Gallantries of the Spaw in Germany. Containing The Virtues of every Spring; their Nature and several Uses. The Reasons why frequented by Persons of the First Distinction, besides drinking the Waters. The Various Diversions and Amusements of the Place. And many entertaining Histories of the principal Persons resorting to Spaw. Intermix’d with several others of Wit, Humour, Gaiety, &c. Translated into English from the French Original (London: Printed for Ward and Chandler, 1737) [Vol. 2, page 192]:

That Cascade has something so majestick, that when I pass’d that Way, added Mr. Lake, I cou’d not help admiring it. The great Inconveniency of it is the horrible Din it makes: for at Lauffen, where I staid to Dine, I cou’d hardly hear what was said to me. And yet the People of the Place are so us’d to it, that they can hear a Pin fall, and speak no louder than in other Places.

Note: In the above-quoted text, “they can hear a pin fall” translates “[ils] entendraient tomber une épingle” (i.e., “they would hear a pin fall”) in the original French text, which is as follows:
—from Amusemens des Eaux de Spa. Ouvrage utile à ceux qui vont boire ces Eaux Minérales sur les Lieux (Amsterdam: Chez Pierre Mortier, 1734), by the German author Karl Ludwig Freiherr von Pöllnitz (1692-1775) [Vol. 2, page 369]:

Cette Cascade à [sic] quelque chose de si majestueux, que quand j’y passai, ajouta Mr. Lake, je ne pouvois me lasser de l’admirer. Ce qu’elle a d’incommode, c’est qu’elle fait un bruit horrible, & qu’étant à Lauffen où je m’arrêtai pour dîner, j’avois peine à entendre ce que l’on m’y disoit. Cependant les habitans dont les oreilles y sont accoutumées entendroient tomber une épingle, & n’y parlent pas plus haut qu’ailleurs.

2-: From The Lincoln, Rutland, and Stamford Mercury (Stamford, Lincolnshire, England) of 24^th May 1793 [Vol. 62, No. 3,246, page 3, column 5]:

LINCOLN, May 14, 1793.

To the AFFLICTED.

CHRISTIAN KREBS, Surgeon and Oculist, begs Leave to inform his Friends and the Public, that he is arrived at Lincoln, where he will continue for a considerable Time. He may be consulted at his Apartments at Mr. Simpson’s, Watch-Maker, Lincoln; he will also attend at Sleaford every Saturday, Sunday, and Monday, and may be consulted at the George Inn; at which Place he will be on Monday the 27^th of this Instant May.
The following Cure has been performed by One Operation, about Twelve Months ago, when Mr. Krebs was at Boston.
Mrs. Sherlock, Wife of Mr. Thomas Sherlock, of Redbonrn [sic], near Brigg, had been very Deaf, so as not to hear the Clock strike, nor the Bell ring; she can now hear a Pin drop, and the lowest Conversation.
N. B. Mr. Krebs begs those troubled with desperate Disorders to apply immediately, and they will not be disappointed.—Letters Post paid.