Green’s Dictionary of Slang

a derog. name for a German, with an implication of stolidity; thus generic for a stolid, unimaginative person.

1914		Ade ‘The New Fable of the Search for Climate’ in Ade’s Fables 68: He would have Otto bring a Tub of the Dark Brew and a Frankfurter about the size of a Sash Weight.
1920		Ade Hand-made Fables 50: Joe did not know that in Germany the Frau caries the Bundles and walks about ten feet behind her beautiful scar-faced Otto.
1931		Ade Old-Time Saloon 95–6: If he worked in one of the sardellen places, it was a safe bet to call him ‘Otto’.
1983		R. Price Breaks 143: Union-card types, a real bunch of Ottos.
2003		Steve Earle Just an Amer. Boy [album] I used to get the shit beat out of me by great big squareheaded cowboys called Otto.