Green’s Dictionary of Slang

(a) to be an incompetent doctor, to work as an itinerant doctor; thus vtr. to subject to medical charlatanry.

1673		M. Stevenson Norfolk Drollery 62: But now he quacks, a Doctor of great skill, / To Cure their bodies, though their souls he kill.
1705–07		N. Ward Hudibras Redivivus II:3 25: All th’ eleven Months beside, / Does Quacking round the Country ride.
1713		W. King York Spy 38: This is the Doctor as I take it, Dress’d up in all that Pomp to Quack it.
1734		C. Johnson Hist. of Highwaymen &c. 173: Having quack’d it from Town to Town as long as he could with Safety, he had a mighty Inclination to come up to London.
1768		Gentleman’s Bottle-Companion 27: See the doctors approach [...] With potions and motions they quack us.
1802		F. Reynolds Folly as It Flies II i: This is the age for quacking, and all clever fellows are at it.
1868		Sportsman 2 July 2/1: Notes on News [...] Whether you have a body to e quacked by Jew nostrum vendors, or a soul to be cured spiritual Sangradoes [etc].

(b) to promote one’s business through fraudulent claims.

1861

(con. 1840s–50s) H. Mayhew London Labour and London Poor I 474/2: If we quacks a bit, does we make fortins by it as shopkeepers does with their ointments and pills!

(a) to complain.

1915		Sport (Adelaide) 19 Feb. 5/1: But do be careful, Butcher, / Be careful how you quack.
1949		N. Algren Man with the Golden Arm 40: You think I come home to hear you quackin’?
1977		L. Heinemann Close Quarters (1987) 123: A bunch of housecatting ARVNs [...] who quacked and cackled and yammered and wouldn’t shut up the whole trip.
2001		N. Barlay Hooky Gear 169: He quack on.

(b) to break wind noisily.

1989

Viz June/July 24: I’ve been experimenting with the quacking qualities of sweets and fizzy pop.

(c) to inform, to betray secrets.

2016

London Rev. Bks 30 June 🌐 You’re gonna have criminal groups that paid him lots of money and there are people who know about that [...] If they quack?