Green’s Dictionary of Slang

treat, a adv.

also treato

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

[UK] ‘’Arry in Switzerland’ in Punch 5 Dec. in P. Marks (2006) 98: I done Parry a treat, mate, this time.
[UK]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.
[UK]A. Binstead Pitcher in Paradise 130: Lordlummy, Harry [...] You don that a treat!
[UK]Magnet 3 Sept. 13: It’s eight inches long, and will do a treat.
[Aus]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.
[UK]Butterfly and Firefly 23 Nov. 1: Marshy and I got on a treato.
[UK]Film Fun 24 Apr. 20: Ho, it goes a treato!
[UK]N. Marsh Death in Ecstasy 231: It’s a rum thing [...] that I can follow that radio bloke a fair treat.
[UK]R. Llewellyn None But the Lonely Heart 115: You want to see him with his teeth in [...] Fit him a treat, they do.
[US]J.P. Donleavy Ginger Man (1958) 68: O I tell you, its coming along a jolly treat.
[UK](con. c.1930) D. Holman-Hunt My Grandmothers and I (1987) 198: It fits a treat.
[UK]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.
[UK]P. Theroux London Embassy 105: A little Brasso and a dry rag – come up a treat.
[Ire]P. McCabe Breakfast on Pluto 182: One thing was certain — I definitely did look a treat.
[UK]N. Griffiths Grits 14: Thuh lasagne’s done a treat, thuh top all bubbly brown an crispy.

In phrases

do one a treat (v.)

to suit one absolutely.

[UK] ‘’Arry in Parry’ in Punch 29 June in P. Marks (2006) 95: The Bullywards do me a treat, mate.
[UK] ‘’Arry on Wheels’ in Punch 7 May 217/2: If you want to try wheels, take the Park; I am sure it’ll do you a treat.