Green’s Dictionary of Slang

1. (Aus./US) a Cornishman.

1885		Bulletin (Sydney) 12 Sept. 11/4: K.G. is an abbreviation common among miners for ‘Cousin Jack.’ K. for Cousin, and G. for Jack.
1890		‘Rolf Boldrewood’ Miner’s Right 92: A short man, whose blue-black curly hair and deep-set eyes betrayed the Cousin Jack.
1899		F. Norris McTeague (1958) 326: The other seemed unsatisfied. ‘Are you a “cousin Jack”?’ The dentist grinned. This prejudice against Cornishmen he remembered too.
1900–10		Stephens & O’Brien Materials for a Dict. of Aus. Sl. [unpub. ms.] 95: We minerrs requirre fowerr meals a day, I have a heard a Cousin jack remark.
1902		Manchester Courier 5 Dec. 14/7: A Cornishman is ‘Cousin Jack’ to the natives of the adjacent counties.
c.1905		J. Furphy Rigby’s Romance (1921) Ch. xiv: 🌐 I told her three Cousin Jacks manhandled me in the (adj.) township.
1910		Bulletin (Sydney) 17 Nov. 14/2: Tom Tommie’s stroke excited great concern at Waddy. It was feelingly discussed at a meeting of the Wesleyan brethren in the tin chapel of which Thomas was an ornament. The Rev. Daniel Glasson went so far as to invite the attendance of seven of the most eloquent Cousin Jacks to a praying bee in his parlor on the following afternoon.
1926		J. Black You Can’t Win (2000) 152: The mines were worked by Irishmen and ‘Cousin jacks’ (Cornishmen).
1930		‘Dean Stiff’ Milk and Honey Route 38: A ‘cousin Jack’ is an Englishman. To the miners he is an Englishman from Cornwall, while a ‘cockney’ is an Englishman from London’s East End.
1938		(con. 1850s) Western Morn. News 29 Aug. 6/3: ‘Cousin Jagger’ is a sailor’s nickname for men of Devon and Cornwall.
1942		‘Cousin Jack’ in Lingenfelter et al. Songs of the Amer. West (1968) 156: You ne’er can beat Cousin Jack [...] For there’s none that can compete / With the good old Cornishman.

2. attrib. use of sense 1.

1885		Bulletin (Sydney) 20 Jun. 7/1: He was sitting demurely in a seat near one of the side windows listening to a stirring dissertation on the trouble in store for the damned, which were luridly described by a Cousin Jack lay preacher, who had previously won a wager of £5 and the championship in a praying contest with the pick of the Kohinoor mullock-punchers.
1900		Bulletin (Sydney) 11 Aug. 14/2: A Cousin Jack Yarn. / Tredinning, miner, Cornishman – / Whose mates this tale assert – / Stepped on a cage, a pasty in / The bosom of his shirt.
1910		Sun. Times (Perth) 19 June 3rd sect. 17/3: Verran, Sou’stralia’s Cousin Jack Premier, is a rabid tea-totaller.
1922		E. Dyson Missing Link 🌐 Ch. x: He dragged to the ground a heroic Cousin Jack miner who was climbing the verandah post.