Green’s Dictionary of Slang

a general term of agreement, indeed! I agree! very much so!

1836		Dickens ‘The Great Winglebury Duel’ in Slater Dickens’ Journalism I (1996) 394: ‘Do you know the mayor’s house?’ inquired Mr. Trott. ‘Rather,’ replied the boots, significantly.
1845		J. Lindridge Sixteen-String Jack 195: ‘It would be summat like a joke to see him here, would it not, young man?’ ‘Rather,’ said Jack.
1846		Swell’s Night Guide 60: Ratherish, my rum’un.
1847		A. Smith Natural History of the Gent 72: Other adopt large noses and false mustaches, which they think is ‘doing it —rather!’.
1858		A. Mayhew Paved with Gold 67: You may go if you like [...] only it’s all found out before this; and won’t you get a licking — rather!
1861		Hills & Plains I 221: ‘Stapleton is fast, I suppose, like the rest of them?’ [...] ‘Rayther; ready to gamble to any extent’.
1866		J. Greenwood Night in a Workhouse 45: ‘Didn’t yer get into a row when you got back?’ some listener asked. ‘Rather! Got kept without dinner and walloped as well.’.
1874		F.C. Burnand My Time 377: ‘Rayther so!’ answered Rowdie, who preferred any slang to the pronunciation [...] of respectable society.
1886		N.Z. Observer and Free Lance (Auckland) 20 Mar. 23/1: Is he struck in that direction? Rather...
1888–94		‘Walter’ My Secret Life (1966) V 991: ‘Has he fucked you Polly?’ ‘Rather,’ the woman replied.
a.1901		Harrington & LeBrunn [perf. Marie Lloyd] Should the Sexes Bathe Together 🎵 They’re [i.e ‘girls’] much safer there - in the sea, you know / Than they are when they’re out of it - rather.
1905		A. Binstead Mop Fair 35: I asked if I should bring a gentleman, she replied enthusiastically, ‘Ray-ther! or his blooming scalp’.
1917		Boy’s Own Paper XL:3 115: Prof. [...] (To Daisy.) You want a situation, I believe? Daisy (pertly). Rather!
1917		A. Brazil Madcap of the School 74: ‘You know your part of the business?’ ‘Ra-ther!’.
1925		Wodehouse Carry on, Jeeves 50: ‘Your aunt said that you would do anything that was in your power to be of assistance to us.’ ‘Rather! Oh, rather. Absolutely.’.
1938		W. Winchell On Broadway 17 Nov. [synd. col.] These artists put London what-ho slang into the mouths of American coal town boys and girls – peasants getting off ‘stout fella’ and the ingenues ‘rahtharhring’.
1947		K. Amis letter 27 Dec. in Leader (2000) 149: ‘Congratulations on your result, Pud! [...] I expect Mum and Dad are pretty bucked, aren’t they?’ – ‘Yes, rather!’.
1950		A. Buckeridge Jennings Goes To School 239: ‘Rather,’ echoed Temple.
1961		‘Frank Richards’ Billy Bunter at Butlins 47: ‘Anything up?’ asked Johnny Bull. ‘Well, yes, rather!’ Bob whistled again.
1976		P. Theroux Family Arsenal 185: ‘You mean you can smell it on me?’ ‘Rather,’ said Lady Arrow; then she sniffed.
1981		G. Young Slow Boats to China (1983) 20: Perhaps my own absurd little smile spoke in the same idiom: ‘Rather!’.
1999		Indep. Rev. 21 June 1: ‘Champagne?’ [...] ‘Rather!’ I reply happily.