*sigh*
Swampland in Florida is a figure of speech referring to real estate scams in which a seller misrepresents unusable swampland as developable property. These types of unseen property scams became widely known in the United States in the 20th century, and the phrase is often used metaphorically for any scam that misrepresents what is being sold.
Swampland in Arizona
A similar phrase, which replaces the Florida with
Arizona, is also used for the same reasons. As Arizona is well known to have an arid climate, it is assumed that wetlands in that state are non-existent.
The implication is that the target of the insult is not only more gullible than someone who would buy swampland in Florida, but also ignorant.