Green’s Dictionary of Slang

a hat with a low crown; primarily worn by carters, it was also popular among the clergy; thus billycock gang, the clergy as a group.

[1721		Amherst Terr. Fil. No. 46. 246: He [the Oxford ‘smart’] is easily distinguish’d by a stiff silk gown [...] a flaxen tie-wig [...] a broad bully-cocked hat, or a square cap of above twice the usual size [OED]].
1838		Leicester Jrnl 2 Nov. 2/7: One of the villains [...] wore a billy cock hat.
1841		Leicester Jrnl 16 July 2/7: Billy Houghton, Esq., captain of the Billy-cock hat brigade.
1846		Covetry Herald 20 Feb. 4/4: He had on a white billycock, trowsers, and a sleeved waistcoat.
c.1849		‘Bloomer Costume’ in C. Hindley Curiosities of Street Lit. (1871) 121: A billycock and feather for / To see which way the wind blows.
1865		Leaves from Diary of Celebrated Burglar 83/2: Dressed up in a country-man’s suit — a brown billy-cock, red neck-‘fogle,’ drab-coloured smock frock, reaching to his ankles.
1871		Australasian (Melbourne) 27 May 5/5: [A]ttiring himself in the very shortest of bobtail coats, the tightest of nether garments, and most resplendent of ‘billycock’ bats, .
1874		‘Old Calabar’ Won in a Canter I 254: ‘No, it is not Mrs. Allsnob, she rides in a topped hat [...] This one has a billy-cock, or a porkpie, or whatever you call it‘.
1879		H. Hartigan Stray Leaves (2nd ser.) 223: [A] billy-cock hat wid a sthreamer av a puggree.
1880		D.W. Barrett Life and Work among Navvies 44: The clergy are playfully spoken of as ‘Billycock Gang’.
1882		G.A. Sala in Living London (1883) Nov. 528: ‘Pot’ hats, ‘wide-awakes,’ and ‘billycocks’ were the almost universal wear among men.
1885		Bulletin (Sydney) 25 Apr. 13/2: No. 2 was a squatocratic magistrate, whose embossed shirt-front was adorned by gold-rimmed studs, and whose billy-cock hat, by its inordinate gloss, struck envy into the heart of the country editor.
1890		Kipling ‘My Great & Only’ in Civil & Military Gaz. 11 Jan. (1909) 265: [used generically] Naturally, the billycocks, seeing what might befall, thought things over again, and you heard the bonnets murmuring softly .
1892		H. Nisbet Bushranger’s Sweetheart 32: A small billycock hat, folded jauntily.
1894		Truth (Sydney) 15 July 1/5: Moleskin pants and billycock, / How at waistcoats white I mock.
1894		A. Morrison Tales of Mean Streets (1983) 29: He wore a billycock with a thin brim and a permanent dent in the crown.
1898		Binstead & Wells A Pink ’Un and a Pelican 227: An old chap in a drab billycock.
1900		Bulletin (Sydney) 7 July 13/3: He had mud-colored whiskers, a billy hat, projecting ‘mickey,’ large teeth, and a voice like a broody hen.
1900		Sporting Times 5 May 1/5: Having tried on a dozen billycocks in succession, and found not one to his liking, the New Broom snarled at the shopman.
1903		A. Binstead Pitcher in Paradise 96: Your worthy parent (here Budds raised his billycock reverently).
1904		Sydney Sportsman (Surry Hills, NSW) 13 Apr. 2/4: Dad would don his coat and billy-cock.
1909		J. Ware Passing Eng. of the Victorian Era.
1926		(ref. to 1823) Western Dly Press 14 Sept. 4/4: The ‘billy cock’ hat received its name from Mr William Coke, who used to wear it at the great shooting parties at Holkham.
1934		T. Wood Cobbers 175: The aboriginals have added new words to the English language [...] drover, billy, never-never.
1949		Monteleone Criminal Sl. (rev. edn).