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Hello. How should I interpret the phrase "an easy mark"?

I came across with this phrase in an article that is written by a man with mantle cell lymphoma, a rare form of blood cancer. The phrase was used in the following sentence: The Internet is a fantastic resource for patients, but some of what I read about mantle cell lymphoma was out of date or even wrong, and logging on began to make me feel anxious. I thought my wife was in denial, and she thought I was an easy mark for every cancer cure.

Thanks in advance.
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A Mark is someone targetted to be cheated.

They call it that because in carnivals, people that appeared to be easy to be cheated were given a chalk mark on their backs to alert the other con-men.
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be easy game/meat (US be an easy mark) SLANG
to be easily deceived:
Old ladies living alone are easy game for con-men.

(from Cambridge Advanced Learner's Dictionary )
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 Vorpar's reply was promoted to an answer.
VorparA Mark is someone targetted to be cheated.

They call it that because in carnivals, people that appeared to be easy to be cheated were given a chalk mark on their backs to alert the other con-men.
Really, the things those people were up to!

Thanks for a funny explanation, Vorpar!
The origin of "easy mark" comes from the Smith Brothers Cough Drops originally manufactured in Pougkeepsie New York. You can look them up in the Wikipedia which gives you the history of the cough drops and how the words "Trade Mark" under the pictures of William and Andrew Smith eventually had people calling William, Trade and Andrew, Mark. Mark was quite a drinker of alcoholic beverages and when he became intoxicated, he would buy drinks and hand out money to his friends. In the morning he wouldn't be able to remember who he had lent money to. Eventually, his friends would intentionally get him drunk and often referred to him as "Easy Mark".
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During the depression, when many 'hoboes' were riding the rails, there was a secret language they used. Besides warnings and directions, another mark designated homes that would provide a meal or handouts to these men.