Green’s Dictionary of Slang

extremely, very much, e.g. gallows poor, very poor.

c.1780		‘The Rover’ in Holloway & Black I (1975) 235: The gallows old whore was always a-growling.
1793		Bacchanalian Mag. 58: ‘Why jealous girls, it’s all my eye — / Beides, tis gallous silly’.
1805		Port Folio 24 Aug. 261: ’Tis gallows silly [HDAS].
1821		Egan Life in London (1869) I 452: You know how gallows lushy she gets.
1827		Egan Anecdotes of the Turf, the Chase etc. 44: [note] Then your Blowing will wax gallows haughty, / When she hears of your scaly mistake.
1835		‘Rampant Moll Was A Rum Old Mot’ in Secret Songster 4: And if you’d know why – most gallowsly / Fond of a drain vos she.
1836		‘Cat’s-Meat Nell’ in Cockchafer 4: Says she, ‘yer gallous polite’.
1836		Berks. Chron. 30 July 4/5: The veather vos so gallus hot.
1837		‘Hurrah For The Mots!’ in Ticklish Minstrel 30: To think they may be [...] gallows hard up vith the pox.
1837		R. Nicholson Cockney Adventures 9 Dec. 43: I’m glad to see yer; we’ve got a gallus large party to-day.
1838		R. Nicholson Cockney Adventures 6 Jan. 76: The youth himself felicitously observed, ‘Most gallusly down on his luck’. [Ibid.] 10 Feb. 116: You’re most gallowsly mistaken, my covies.
1840		Flash Mirror 8: You get gallows saucy now.
1842		‘The Charming Mot’ Nobby Songster 41: Some say she was once in high keeping / But lately she’s grown gallus poor.
1843		T. Haliburton Sam Slick in England I 19: You are among strangers, formal, cold, gallus polite, and as thick in the head-piece as a puncheon.
1850		Manchester Spy (NH) 5 Oct. n.p.: The Gof-fes-town Light infantry, a ‘gallus’ looking country company.
1851		Manchester Times 23 July 8/1: In the last generation it was a popular notion with the working classes of London, that ‘the shires (or sheers as they call them) were a gallos long way off’.
1853		T. Haliburton Sam Slick’s Wise Saws I 153: He goes out to meet him, gallows polite.
1862		H. Kingsley Ravenshoe II 163: And the pleece come in, and got gallus well kicked about the head.
1865		‘Oh! What A Flare-Up’ in Rambler’s Flash Songster 34: I never was in such a strong place afore, / And I’ll take gallows good care, I go there no more.
1873		R. Broughton Nancy III 187: How gallus bad their ’taters were last year.
1877		J. Greenwood Dick Temple II 237: ‘I am gallows hard up for capital.’.
1889–90		Barrère & Leland Dict. of Sl., Jargon and Cant.