Green’s Dictionary of Slang

spiffingly adv.

extremely well.

1890		‘’Arry on [...] the Glorious Twelfth’ in Punch 30 Aug. 97/2: My rig-out was a picter [...] / I shaped spiffin, in mustard-and-mud-colour checks.
1946		(con. 1914) B. Marshall George Brown’s Schooldays 215: I say, Marigold, and isn’t he spifflingly handsome?
1999		K. Sampson Powder 225: Feedback from Hannah was that last week’s interviews had gone spiffingly.
2001		(con. 1945–6) P. Doyle Devil’s Jump (2008) 161: ‘I think we’re going to get on splendidly, Bill, don’t you?’ ‘Yeah, absolutely spiffingly.’.