Green’s Dictionary of Slang

(orig. US) a general excl. of disgust or disbelief.

1886		Lantern (N.O.) 20 Oct. 5: What a rotten game. Rats!
1890		Sporting Times 1 Feb. 1/5: In conclusion, ‘May the best men win!’ (Yah! Get out! What are you giving us? Rats! Chestnuts! It’s been said before!).
1894		‘O. Henry’ ‘A Successful Political Intrigue’ in Rolling Stones (1913) 151: ‘She is very beautiful,’ says Luderic. ‘Rats,’ says Mabel.
1903		A. Binstead Pitcher in Paradise 263: Rats on yer Caffy Royal to begin with!
c.1905		J. Furphy Rigby’s Romance (1921) Ch. viii: 🌐 ‘Rats!’ says Parryo [i.e. Pharaoh]. ‘Gorstruth!’ says he, ‘did you think you’d come Paddy over me? Won’t wash no (adj.) road.’.
1910		C. Meredith Peggy of Cape Town 89: ‘Rats!,’ said his wife, lighting a cigarette, and surveying him with an air of insouciance.
1911		Truth (Sydney) 1 Jan. 8/3: ’Rats!’ his mate sez; ‘that there puggy / Nose ain’t yourn, as I can see!’.
1914		S. Lewis Our Mr Wrenn (1936) 69: Oh, rats! Say, I didn’t mean to get huffy.
1922		Joyce Ulysses 490: virag: (Sloughing his skins, his multitudinous plumage moulting.) Rats!
1922		R. McAlmon ‘A Boy’s Discovery’ in A Hasty Bunch 62: Rats, that’s a fairy story like Santa Claus.
1924		C.J. Dennis ‘The Faltering Knight’ in Chisholm (1951) 73: But when ’er look sez ‘Rats!’ where’s the reply / A man can give, an’ keep ’is dignity?
1933		C. Drew ‘Sledgehammer Joe’ in Bulletin (Sydney) 19 July 48/3: ‘Rats!’ he snorts. ‘If I’d had any sense under the bone that carries me hat I’d never have let meself get carried here’.
1936		Hotspur 11 Jan. 45: Rats and more rats! It’s American.
1945		D. Bolster Roll On My Twelve 107: And Rats to you!
1950		A. Buckeridge Jennings Goes To School 215: ‘Oh, rats!’ he replied. ‘Those are cake crumbs.’.
1961		‘Frank Richards’ Billy Bunter at Butlins 163: ‘Rats!’ said Bunter.
1972		P. Fordham Inside the Und. 150: Saying rather rudely ‘Rats!’ to some unusually silly piece of information.
1989		H. Rawson Dict. of Invective (1991) 323: Rats! is an expression of annoyance or disgust.
1999		Indep. Rev. 4 Sept. 20: Rats, I thought, she must have missed her train.