Green’s Dictionary of Slang

1. a fool, a dullard; also a term of address.

1932		R. Fisher Conjure-Man Dies 13: All right, dumbo. Run away and prove you done it.
1941		Walt Disney Prods. [film title] Dumbo.
1959		S. Aleichem My First Love Affair and Other Stories (2002) 89: But he was called Dumbo because he was, to put it plainly, a boor, a country bumpkin, a tall and powerful hick.
a.1964		‘Dumbo the Junkie’ in D. Wepman et al. Life (1976) 89: In walked Dumbo the junkie, dripping wet from the rain.
1975		‘P.B. Yuill’ Hazell and the Three-card Trick (1977) 63: Sorry for him? Me? Sorry for any dumbo who strolls into the West End and thinks he can enjoy an honest game of chance?
1980		Maledicta IV:2 (Winter) 227: Dora (Dumb Dora) and Dumbo or Dumbellina (from Disney’s cartoon elephant and dumbbell) are heard.
1990		T. Winton Human Torpedo 121: I haven’t got a Cherry Ripe, dumbo.
1999		Guardian Guide 3–9 July 9: I am, however, a great big dumbo.
2013		B. Leopold ‘With One Stone’ in ThugLit Apr. [ebook] ‘That’s [i.e. a gang boss] who you’re looking out for, Dumbo’.
2024		J. Meades Empty Wigs (t/s) 298: ‘What did he meant by that Mister Ricky? Keep... yourself... buoyant...’ ‘Pwhh...’ I played dumbo.

2. a foolish blunder.

1952

Sat. Eve. Post 20 Sept. 40: If you think you’ve seen dumbos pulled on the highways, you haven’t seen anything [W&F].