Green’s Dictionary of Slang

excellently, wonderfully.

1849		‘Epistle from Joe Muggins’s Dog’ in Era (London) 20 May 4/1: It would be summut out of the common way if sich an animal [...] was to win, wouldn't it? I can only tell you I saw him, and he goes stunnin.
1854		F. Smedley Harry Coverdale’s Courtship 302: He be a-getting on stunnin’, Mr. Philip.
1861		(con. 1840s–50s) H. Mayhew London Labour and London Poor I 204/2: It was rolled in a ha’penny stick, sir, and sold stunnin’.
1871		C. Hindley Curiosities of Street Lit. 51: He [...] ‘did stunning, until he was afraid of being stunned on skilly’.
1875		Auckland Eve. Star (Supp.) 30 Oct. 6/2: Quite so [...] Stunning jolly, isn’t it?