One of the things I love about FR is that I learn something every time I log on!
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According to HistoryBuff.com, you are correct:
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OTOH, (years ago) I read somewhere that ol' P.T. adopted and adapted the saying after it was directed at him. His version supposedly went:
P.T.B.: "There's one born every minute!"
Sucker: "One what?"
P.T.B.: "Hello, there, Sucker!"
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True or not, it (like the "Egress" gag) sounds like something PhineasT. would have delighted in doing... '-)
I wouldn't be suprised at all to learn that he took the phrase and turned it to his advantage. :)