Green’s Dictionary of Slang

1. a person whose thoughts and conversation are conventional and commonplace; a dull, boring person.

1904		Topeka State Jrnl 2 Mar. 8/5: The Bromide and the Sulphite People. The typical bromide — and all bromides are typical — is fond of discussing ‘symptoms’ [...] Most women are bromides [...] Boys are excessively bromidic also.
1906		G. Burgess Are You A Bromide? 17: The Bromide does his thinking by syndicate. He follows the main-travelled road, he goes with the crowd. In a word, they all think and talk alike.
1909		W. Irwin Confessions of a Con Man 126: An average-minded man – what they’re calling a ‘bromide’ these days.
1926		Wood & Goddard Dict. Amer. Sl.
1934		S. Beckett More Pricks than Kicks 64: He was run plump into by one Chas, a highbrow bromide of French nationality.
c.1956		‘Flann O’Brien’ ‘Fossilanguage’ in Hair of the Dogma (1989) 80: I have been looking at a little book [...] it is by an American, Gelett Burgess, and deals with ‘bromides’.

2. (also bromidiom) a commonplace saying, a trite remark; a soothing statement; thus bromidic, bromidically, commonplace, conventionally.

1906		G. Burgess Are You A Bromide? 17: It is not merely because this remark is trite that it is bromidic.
1920		Ade Hand-made Fables 121: He pulled a weak Bromide about Liberty Hall but, just as it escaped him, he realized that it sounded Hollow and Unconvincing.
1929		S. Ornitz Haunch Paunch and Jowl 263: ‘Good citizenship should not condone wrongdoing.’ I have repeated his memorable bromidiom.
1932		V.F. Nelson Prison Days and Nights 183: It may be very bromidically said that the criminal is, after all, a human being.
1933		J. Conroy Disinherited 188: I almost let the bromide about ‘two’s company, three’s a crowd’ slip out.
1942		in W.C. Fields By Himself (1974) 414: The Christian Science Monitor: Day in and day out the same old bromides.
1961		F. Kohner Gidget Goes Hawaiian 43: She already gave me that bromide – and it didn’t fizzle.