Green’s Dictionary of Slang

a general intensifier, well, satisfactorily or very.

1824		in Spirit of Public Journals (1825) 347: They’ve served me pretty tidy going along, [...] punching at me with their shilaleaghs as they would at a woolsack .
1824		Pierce Egan’s Life in London 26 Sept. 5/2: His Lordship rushed in with great impetuosity, and placed two feelers on Matthew's proboscis, which uncorked the claret tidily.
1858		A. Mayhew Paved with Gold 92: I shall buy a pound of bread [...] because I’ve done pretty tidy.
1861		(con. 1840s–50s) H. Mayhew London Labour and London Poor III 29/1: When I first took to silling ’em, there was a tidy lot of boys at the business. [Ibid.] IV 428/1: Them old chaps there seems to do it pretty tidy.
1873		Sl. Dict. 322: Tidy tolerably, or pretty well; ‘How did you get on to-day?’ ? ‘Oh, tidy.’.
1887		‘’Arry at the Sea-Side’ in Punch 10 Sept. 111/1: Although, entry noo, dear old pal, it’s a tidy stiff drain on yer ‘screw.’.
1899		R. Whiteing No. 5 John Street 92: ‘Was you knocked about much when you was a young ’un?’ ‘Pretty tidy, only I alwiz stepped it when it got too ’ot.’.
1974		(con. 1940s) E. Thompson Tattoo (1977) 55: Between genuinely wanting to keep Gene tidy and beating him out of a dollar or two a night, they could not resist making [...] fun of him.