Green’s Dictionary of Slang

bumper adj.

[bumper n.2 (3)]

especially large, especially abundant.

[UK]R.S. Surtees Ask Mamma 285: It is going to be a bumper meet, for the foxes are famous.
[Aus]Bell’s Life in Tasmania 13 Sept. 3/3: If ever a ‘bumper house’ were deserved on a benefit night, the present occasion merits it doubly.
[Aus]Bulletin (Sydney) 7 Mar. 16/3: Mons. de Willimoff’s benefit is announced to come off to-morrow (Thursday) night, when I truly hope he will make a bumper house.
[UK]Music Hall & Theatre Rev. 16 Feb. 4/2: A bumper gate turned up to support him.
[Aus]Bird o’ Freedom (Sydney) 21 Feb. 3/2: [H]e intends to give the big fellow a bumper benefit.
[UK]Boy’s Own Paper 16 Feb. 306: I shall soon be a bumper ship with a bing on deck amidships too.
[US]J.W. Carr ‘Words from Northwest Arkansas’ in DN III:ii 129: bumper crop, n. phr. An unusually large crop. ‘It is a common thing in Washington and Benton counties to get a bumper crop of apples.’.
[US]L.W. Payne Jr ‘Word-List From East Alabama’ in DN III:iv 295: bumper, adj. Very large, full. ‘We are raising a bumper crop this year.’.
[Ire]Joyce ‘A Mother’ Dubliners (1956) 138: The Friday concert was to be abandoned and the committee was going to move heaven and earth to secure a bumper house on Saturday night.
[UK]T. Norman Penny Showman 69: And what a bumper weekend we had to be sure.
[Aus]K.S. Prichard Haxby’s Circus 235: A bumper audience in Cobar filled the till.
[UK]J. Curtis There Ain’t No Justice 33: The management has decided to add a further contest to a bumper bill.
[UK]J. Lees-Milne Ancestral Voices diary 2 Apr. (1975) 178: Johnny Dashwood lunched with me at Brooks's. An expensive guest for he had a gin and tonic, a whisky and soda and a bumper brandy away.
[UK]P. Larkin letter 7 Dec. in Thwaite Sel. Letters (1992) 231: Well, that’s certainly a bumper news budget!
[Ire]J. Morrow Confessions of Proinsias O’Toole 97: I went to the bar and poured two bumper buckshee Powers.
[UK]Guardian Guide 21–27 Aug. 26: Hoddinot [...] has enjoyed something of a bumper year in Wales.
[UK]Indep. Rev. 29 Jan. 20: I’m sure I can provide you with a bumper one at the weekend.