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Hans Zinsser: As I Remember Him (1940, Little, Brown and company) No rating

This work is an autobiography told in the third person, and includes coverage of his …

For prose, scientific or otherwise, as for riding horses, he believed one should be start sober; when he was writing essays on educational subjects, he felt that a spot of beer put him into the solemn-ass mood and thus a little closer to the state of mind of the professional pedagogue.

As I Remember Him by  (Page 291)

my whole life is shaken seeing "-ass" used like this in a book published in 1940

@mouse @tikitu Calvin Coolidge referred to himself as a “solemn ass” in I’m guessing the 1920 campaign. I see a Van Dyke poem about a solemn ass in 1906 and one shows up in “The Ass and the Brock,” by Allan Ramsay (1686-1758).

That’s all in the donkey sense. In British English ass only means donkey, the other thing is an arse. Not sure when it became ambiguous in America.

It’s hyphenated because it’s an adjectival noun phrase, like “old-boy network”