Green’s Dictionary of Slang

wonderfully, extremely, excessively, e.g. that’ll go down a treat.

1889		‘’Arry in Switzerland’ in Punch 5 Dec. in P. Marks (2006) 98: I done Parry a treat, mate, this time.
1895		Merion & Raynor [perf. Dan Leno] ‘Bow Street’ 🎵 ‘Wait a moment, witness, do you know the nature of an oath?’ Oh! that kid did slam a treat.
1903		A. Binstead Pitcher in Paradise 130: Lordlummy, Harry [...] You don that a treat!
1910		Magnet 3 Sept. 13: It’s eight inches long, and will do a treat.
1918		C.J. Dennis ‘Before the War’ in Digger Smith 15: First off, young Jim, ’oo plays it ’igh a bit, / Narks the ole man a treat.
1918		Butterfly and Firefly 23 Nov. 1: Marshy and I got on a treato.
1920		Film Fun 24 Apr. 20: Ho, it goes a treato!
1936		N. Marsh Death in Ecstasy 231: It’s a rum thing [...] that I can follow that radio bloke a fair treat.
1943		R. Llewellyn None But the Lonely Heart 115: You want to see him with his teeth in [...] Fit him a treat, they do.
1955		J.P. Donleavy Ginger Man (1958) 68: O I tell you, its coming along a jolly treat.
1960		(con. c.1930) D. Holman-Hunt My Grandmothers and I (1987) 198: It fits a treat.
1979		F. Norman Too Many Crooks Spoil the Caper 68: The kidnapping had gone off a treat: bang crash wallop, and the girl was in the car and gone.
1982		P. Theroux London Embassy 105: A little Brasso and a dry rag – come up a treat.
1998		P. McCabe Breakfast on Pluto 182: One thing was certain — I definitely did look a treat.
2000		N. Griffiths Grits 14: Thuh lasagne’s done a treat, thuh top all bubbly brown an crispy.