Green’s Dictionary of Slang

second-rate, inferior, ‘rubbishy’.

1827		Pierce Egan’s Life in London 15 Apr. 926/1: [A] number of the ‘two-penny halfpenny,’ or ‘Cag Mag,’ publishers advertised it [i.e. a song sheet] at a fourth of the price set down by the holder of the copyright.
1833		‘Ar’nt This Rummy Fun?’ in Fun Alive O! 62: For I will prove as you shall larn, I’m no cag-mag consarn!
1836		London Dispatch 4 Dec. 3/3: the Wapping coalheavers [...] professed themselves so affronted at his ‘cagmag’ dainties, that they seized his pie-oven and rubbed his nose in his own pies.
1849		Sam Sly 24 Mar. 2/1: Sir W—m O—r, not to sell his cag-mag meat on pretence of philanthropy.
1859		G.A. Sala Twice Round the Clock 274: The fumes of [...] ancient fish, of cagmag meat, of dubious mutton pies [...].
1861		(con. 1840s–50s) H. Mayhew London Labour and London Poor III 253/1: It’s no use a publican trying to gammon us with any of his cag-mag stuff.
1861		Dickens Great Expectations (1992) 156: Oh Jaggerth, Jaggerth, Jaggerth! all otherth ith Cag-Maggerth.
1864		Temple Bar X 185: No kag-mag wares are sold, no cheap articles are retailed [F&H].
1876		J. Greenwood Low-Life Deeps 219: It would be a reg’lar insult to the stars to go to ’em and consult ’em at such a cag-mag price.
1883		J. Greenwood Tag, Rag & Co. 103: Afore I’d work for a cussed three fardens a hour and the cag-mag kind of wittels that you’ve been eating, svelp me if I wouldn’t turn highway robber.
1890		Dundee Advertiser 29 Mar. 5/1: The nefarious traffic of the ‘carrion’ or ‘cagmag’ butcher [...] is carried on in certain quarters.
1901		Sydney Sportsman (Surry Hills, NSW) 10 Apr. 1/1: Some of the capers cut by this cag-maggy crowd are very suggestive.
1927		Western Mail (Perth) 10 May 27/4: Anything Dowdy [...] It is ‘cagmag,’ a variation is ‘cagmaggy’.
1936		Sun. Times (Perth) 21 June 11/2: When we eat the grub of garrets— / Cag-mag meat, with spuds and carrots.