"Oh, I ain't no Guy de Mopassong," he said, cheerfully. "I'm giving it to you in straight American. Well, she says something like this: 'Mr. Gold Bonds is only a ...
Top answer
org/wiki/Guy_de_Maupassant
— Fivejedjon
org/wiki/Guy_de_Maupassant
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It's probably a play on the name of the French writer http://en.wikipedia.org/wiki/Guy_de_Maupassant. The person speaking is boasting that his language is not complicated, but rather "straight American." It is ironic because "American" isn't a language.