Green’s Dictionary of Slang

wonderful, perfect, thus adv. splendiferously.

1538		Bale Enterlude Johan Baptystes in Harleian Misc. I (1808) 113: O tyme most joyfull, daye most splendiferus. The clereness of heaven now appeareth unto us.
1837		R.M. Bird Nick of the Woods I 174: Oh! you splendiferous creatur’! you anngeliferous anngel!
1843		T. Haliburton Sam Slick in England I 216: Even Squire Merton and his splendiferous galls walked like the poorest of the poor.
1853		T. Haliburton Sam Slick’s Wise Saws I 243: We will then have a splendiferous spree, uncork it, and get riproarious with delight.
1859		G.A. Sala Twice Round the Clock 122: As to their attire, it is positively — if I may be allowed the use of a barbarism — ‘splendiferous.’.
1863		C. Reade Hard Cash II 188: I see the splendiferous articles arrive, and then they vanish forever.
1876		C. Hindley Life and Adventures of a Cheap Jack 86: It’s a splendiferous silver guard-chain.
1882		G.A. Sala in Living London (1883) Nov. 505: I confess that until a very little while ago I thought that ‘splendiferous’ was a slang word, of purely modern and ‘Arry-an’ invention.
1892		‘’Arry in Venice’ in Punch 27 May 88/1: The scenes was splendiferous.
1898		W.A. Sun. Times (Perth) 16 Jan. 7/1: [He] penned the following cable [...] ‘Splendiferous-cheesecake-oderous -marsupial-polecat-soapwell’.
1905		‘Central Connecticut Word-List’ in DN III:i 20: splendiferous, adj. Splendid.
1908		Sporting Times 25 Apr. 4/4: ‘Stelliferous and Splendiferous,’ as per my prediction last week, is the Easter entertainment at the Oxford.
1914		A. Brazil Leader of the Lower School 255: ‘No more gadding about the world just at present. Will that suit you, little woman?’ ‘Splendiferously!’ answered Gipsy.
1917		A. Brazil Madcap of the School 257: ‘Splendiferous! I’ll do it!’ she said aloud;.
1929		J.B. Priestley Good Companions 607: ‘But honestly, isn’t it marvellous?’ ‘Splendiferous!’.
1941		B. Miller Farewell Leicester Square (2000) 251: The train was flying along between rows of tall brick houses; between back-gardens unequally mean or splendiferous.
1949		Sat. Eve. Post 5 Mar. 100/2: Inside are what must have been among the most splendiferous public rooms of that splendiferous period [DA].
1952		Lait & Mortimer USA Confidential 158: Tommy Banks moved in, opening the splendiferous Horse Shoe Club.
1978		J. Krantz Scruples 359: He was a splendiferous one-man band.
1987		J. Ciardi Good Words 141: Great. Groovy. The max. Splendiferous.