The term savoury duck denotes a faggot, a meatball. This term seems to have originated in Lancashire, a county of north-western England, on the Irish Sea.

The following, from The Liverpool Echo (Liverpool, Lancashire, England) of Friday 20^th August 1880, mentions the ingredients of a savoury duck—Bolton is a town historically situated in Lancashire:

“SAVOURY DUCKS.”

The medical officer of health of the borough of Bolton, in his monthly report to the sanitary committee, sounds a note of warning to fanciers of the bonnes bouches commonly known in Lancashire as “savoury ducks”—a compound of onions, flour, and small pieces of pork. The doctor says that on the 31^st ult. one of the sanitary inspectors of the borough found “the hind quarter a rat” in one those delicious morsels. “Further examination,” he says, “showed that the rat might have accidentally got among the food during the preparation, but even so, it is very disgusting; and I would advise people who prefer this style of bonne bouche to be, in future, more critical in exploring the contents of these savoury articles.”

The term savoury duck is probably humorous, and similar to nouns such as:
– Bombay duck, denoting the bummalo fish, especially when dried and eaten as an accompaniment to curries;
– Welsh rabbit, denoting a savoury dish consisting of melted cheese sometimes mixed with milk, seasonings, etc., on hot buttered toast.

The reason such items of food were so named was explained by the Reverend Abram Smythe Palmer (1844-1917) in Folk Etymology: A dictionary of verbal corruptions or words perverted in form or meaning, by false derivation or mistaken analogy (London: George Bell and Sons, 1882)—French poulet de carême translates as Lent chicken:

The phrase [Welsh rabbit] is one of a numerous class of slang expressions—the mock-heroic of the eating-house—in which some common dish or product for which any place or people has a special reputation is called by the name of some more dainty article of food which it is supposed humorously to supersede or equal. Thus a sheep’s head stewed with onions, a dish much affected by the German sugar-bakers in the East-end of London, is called  “a German duck;” a Leicestershire Plover is a bag-pudding (Ray); a species of dried fish is “a Bombay duck” in Western India; a crust of bread rubbed with garlic is in French slang “a capon;” in Cambridgeshire cow-heel is “a cobbler’s lobster” (Wright); red herrings are variously known as “Norfolk capons,” “Dunbar wethers,” or “Gourock hams.” “Sheep’s head” is an old name for a Virginian fish from which something like mutton broth could be made (Bailey). “Mummers’ feed is a herring which we call a pheasant,” says a strolling actor in Mayhew’s London Labour and London Poor, vol. iii. p. 151. In French it is popularly called poulet de carême. A cheap dish composed of liver, potatoes, &c., is termed “a poor man’s goose.” Similarly a dish of roasted cheese was regarded as the Welshman’s rabbit.

Likewise, this is what the British anthropologist Edward Burnett Tylor (1832-1917) explained in The Philology of Slang, published in Macmillan’s Magazine (London: Macmillan and Co.) of April 1874:

Welsh rabbit is a genuine slang term, belonging to a large group which describe in the same humorous way the special dish or product or peculiarity of a particular district. For examples: an Essex stile is a ditch, and an Essex lion a calf; a Field-lane duck is a baked sheep’s head; Glasgow magistrates, or Gourock hams, or Norfolk capons, are red herrings; Irish apricots or Munster plums are potatoes; Gravesend sweetmeats are shrimps; and a Jerusalem pony is a donkey.

And the following is from Americanisms—Old and New. A Dictionary of Words, Phrases and Colloquialisms peculiar to the United States, British America, the West Indies, &c., &c., their Derivation, Meaning and Application, together with Numerous Anecdotal, Historical, Explanatory, and Folk-Lore Notes (London: Privately printed by Thomas Poulter & Sons, 1889), by the British lexicographer John Stephen Farmer (1854-1916):

Marble-Head Turkey.—A Massachusetts term for a cod-fish. Also called Cape-Cod turkey. There are many instances of fish being spoken of as meat, e.g., the sturgeon is known in America as Albany beef; while in England herrings are nicknamed “digby chicks” when dried, or “Billingsgate pheasants” when fresh; and a Yarmouth bloater rejoices in the euphonious name of “two-eyed steak.” Many other examples will occur to mind as colloquial on both sides of the Atlantic.

These are, in chronological order, the earliest occurrences of the term savoury duck that I have found:

1-: From the account of a case brought before the court leet for the township and manor of Manchester, published in The Manchester Guardian (Manchester, Lancashire, England) of Saturday 19^th October 1833:

Mr. Reuben Gill, pork-dealer, of St. Mary’s Gate, was then presented for a nuisance caused by the stench emitted from his shop and cellar, chiefly in the preparation of sausages, “savoury ducks,” and other articles of sale.

2-: From letters to the Editors, by “a burgess of Manchester”, published in The Manchester Times. And Lancashire and Cheshire Examiner (Manchester, Lancashire, England):

2.1-: Of Saturday 28^th November 1835 (letter dated 24^th November 1835):

Gentlemen:—[…] The town is daily increasing in magnitude and in numbers; more space is required for its traffic and the loco-motion of its people; large sums of money, raised among the inhabitants, are annually expended by the commissioners for the providing of this space; yet in defiance of the commissioners, to the great inconvenience, loss and danger of the public, the protege of Sir Oswald Mosley step in, erect on the “foot-paths and carriage-road” their cooking apparatus for savoury ducks, as they are called, their stalls for tripe, trotters, cow heels, old iron, worm powders and worms, and all other filth and garbage which can incommode a people or disgrace a town. […]
[…] The Lord of the Manor, I am informed, paid a visit to Manchester and met a deputation from the police body last week. At this meeting he impressed on the deputation a due sense of his power. He told them that he had incurred great expense in improving and adorning the town, and expressed his readiness on all occasions, when consistent with his duty, to forward the object of the Commissioners in their public improvements. […] Allow me to proclaim to your readers the important fact that Sir Oswald Mosley promised the deputation that the shop of the “savoury duck” man should be removed from the foot-path, and to congratulate them on the distant prospect, should they only be patient, of the quack doctors, the worms, and the worm powders, being also removed. The tripe, trotters, cow-heels, and other garbage, must remain as evidence of Sir Oswald’s condescension and respect for public rights and public accommodation.

2.2-: Of Saturday 5^th December 1835 (letter dated 30^th November 1835):

Gentlemen:—The footpath from Bradshaw-street to Snow-hill is cleared. The savoury ducks, the tripe, the cow-heels, the quack-doctors and their compeers—the worms, are all removed, and my prophecy is more than fulfilled.

2.3-: Of Saturday 26^th December 1835 (letter dated 22^nd December 1835):

Gentlemen:—The surveyors of the highways have resolved that the streets shall be no longer occupied, nor the safety of the people endangered by stalls placed alongside the footpaths for the benefit of the lord of the manor. For this resolution they deserve, and shall receive, the thanks of the inhabitants of Manchester.
The patron of the pickled-pigs’ heads, of the pudding man, of the savoury ducks, of the worms and worm doctors, must remove the garbage to his market, should he think fit to disgust those who attend his market by such exhibitions.