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The expression mauvaise langue (plural mauvaises langues) designates:
– a vituperative gossip, a scandalmonger;
– an evil or malicious tongue—as in quotation 5 below.

This expression occurs, for example, in the following from State chiefs’ heads on the block, published in the Evening Standard (London, England) of Wednesday 11^th November 1992 [page 32, column 1]:

Business is politics in a very literal way for the heads of the French State companies, many of whose heads may be rolling if next spring’s legislative elections go the wrong way.
As the gossips have it (or mauvaises langues as they tend to be called), industry chiefs have been currying favour with candidates likely to inherit the mantle of power next March.

This expression is a borrowing from French mauvaise langue, of same significations, composed of:
– the adjective mauvaise, feminine of mauvais, meaning bad;
– the feminine noun langue, which here designates the tongue.

—Cf. also the expressions a bad quarter of an hour (loan translation from French un mauvais quart d’heure) and mauvais coucheur.

These are, in chronological order, the earliest uses of the expression mauvaise langue that I have found in English texts:

1-: From a correspondence from Paris, published in The Morning Chronicle (London, England) of Monday 23^rd December 1822 [page 2, column 5]:

The Members of the Royal Family, who are constant inmates of the Pavillon Marsan ¹, pleading poverty, got six months of their allowances paid them in advance, and yet at the end of the month some of their tradesmen did not receive payments they had a right to expect. Des mauvaises langues instantly spread the report that the whole of the money advanced had travelled on to the Pyrenees, and that M. Ouvrard’s ² loan was invented as a cloak for this dexterous operation.

¹ The Pavillon de Marsan was part of the Tuileries Palace, the Parisian residence of French monarchs.
² Gabriel Julien Ouvrard (1770-1846) was a French financier.

2-: From A Soiree at Stupinis, published in The Morning Post (London, England) of Friday 13^th May 1842 [page 3, column 6]—this text is apparently a translation of an article by one P. D’Arrieux published “in a Paris Print”:

Stupinis is to Turin what the Chateau of St. Cloud is to Paris. The same distance separates those two Royal residences from the respective capitals. The stay of Prince Borghese and his wife at Stupinis gave rise to some occurrences which my discretion had better omit, and revived likewise some ultra-gallant anecdotes connected with certain rooms in a long passage to the left, at the period of Bonaparte’s sojourn, previous to the Milan coronation. These scandalous adventures must be left to mauvaise [sic] langues, and they who are not such will do as I do, shut their eyes when they have seen all, and stop their ears when they have heard all.

3-: From Foreign News, published in The Daily Union (Washington, District of Columbia, USA) of Saturday 5^th July 1845 [page 218, column 5]—reprinted from the New York Courier and Enquirer (New York City, New York, USA) of Friday 4^th July 1845:

The principal piece of gossip in the salons of Paris, has been the recent marriage of a Madame de St. Mars, better known as the author of one or two romances, under the name of the Comtesse Dash ³, with the son and heir of the reigning prince of some part of the civilized region of Moldavia. The bridegroom is twenty-two and the lady forty-four. The young man, according to the tale in the salons, ran away with the lady; but the mauvaises langues—of which there are as many in Paris as elsewhere—insist that it was the lady that ran away with him.

³ Comtesse Dash (or D’Ash) was one of the pseudonyms of Anne de Cistemes de Courtiras, Marquise (or Vicomtesse) de Poilloüe de Saint-Mars.

4-: From The Freeman’s Journal, and Daily Commercial Advertiser (Dublin, County Dublin, Ireland) of Tuesday 20^th January 1846 [page 2, column 3]:

ASTOUNDING, IF TRUE.

We are informed upon authority which we have no reason to discredit, that a carriage bearing the arms of the Heytesbury family was “actually seen” in Sackville-street upon one occasion last week. The malicious insinuations of some “mauvaises langues,” who pretended that we were without a Lord Lieutenant, are now set at rest.—The carriage, and not a covered car, as was first asserted, was drawn by “two horses.” His Excellency’s stud is, therefore, not quite “unique.”

5-: From the following letter, by ‘Argus’, published in The Satirist: Or, The True Censor of the Times (London, England) of Saturday 26^th August 1848 [page 357, column 3]:

DOINGS AT OSTEND.
TO THE EDITOR OF THE “SATIRIST.”

Ostend, 20^th August.
Dear Sat—Every year I send you some account of the foreign bathing-places, and having visited them all shall do so this season. Baden, Wiesbaden and Homberg are utterly neglected; Spa has a tolerable sprinkling, but all invalids; there is no play at all, and the gamblers are at their wits’ end. Ostend is the only place gaining any harvest; here has been a constant succession of visitors since the beginning of July, amongst them the Duc Darmstadt, Prince Sapio, Prince Pierre D’Aremburg, [&c.]: all these people unite in everything for the benefit of the town, and are, of course, more popular than the English. Amongst the latter are a lot of nobodies, Sutton, a patent medicine man, and Attlies, the distillers, doing it very fine. A family named Bedford; a retired grocer named Thomas, such a mauvaise langue as that man has cannot be told!