Green’s Dictionary of Slang

1. an idiot, a dullard, esp. an old one; thus dodo brain n., an idiot.

1898		F. Norris Moran of the Lady Letty 55: Shut up, you crazy do-do.
1900		Flynt & Walton Powers That Prey 32: They called me a ‘jaundiced tutelary dodo’.
1915		T.A. Dorgan Indoor Sports 22 Jan. [synd. cartoon] Can’t that dodo spill the chin goods.
1928		R. Carr Rampant Age 199: Paul referred to him as a ‘moss-backed dodo’.
1933		J. Gray ‘The Nudist Gym Death Riddle’ in Vice Squad Detective 🌐 Half a dozen flabby old dodoes who were still running in circles, unable to decide where to go.
1978		L. Kramer Faggots 71: A dumb dodo of a daddy.
1989		H. Rawson Dict. of Invective (1991) 13: Birds are particularly popular in the lexicon of animal derogation, especially as insults for [...] anyone who is lacking in brains, i.e., as dumb as a dodo, as crazy as a coot, cuckoo, or a dupe.
1991		J. Osborne Déjàvu Act I: I exposed the vicious oik struggling inside every carping old dodo like J.P.
1997–2002		Hope College ‘Dict. of New Terms’ 🌐 do-do brain n. Someone who isn’t that smart.

2. a conservative, one who refuses to change with the times; also attrib.

1874		L. Carr Judith Gwynne n.p.: He belongs to the Dodo race of the real unmitigated . . . Toryism [R].
1886		H. Goldsmid Dottings of a Dosser 111: The best way to ensure such an object is to entrust the execution of the measure to ‘the Dodo of Scotland Yard’.
1909		J. Ware Passing Eng. of the Victorian Era 113/1: Dodo (Amer., beginning to be known in England). A human fossil, a man who clings to the past, and condemns future days and present [...] Dodo (Press, 1885). Scotland Yard – figuratively to express that the metropolitan police were fossil in their organisations.
1922		F.S. Fitzgerald letter 18 June in Letters (1964) 164: Tom Boyd wrote me that Bridges had been a dodo about some Y.M.C.A. man.

3. (US drugs) a drug addict.

1935		A.J. Pollock Und. Speaks 32/1: Dodo, a narcotic addict; dopefiend; hophead.
1950		P. Hoskins No Hiding Place! 190/1: Dodo. A dope addict.
1953		Anslinger & Tompkins Traffic In Narcotics 307: dodo. A drug addict.