…what a “merkin” is: “A Merkin” has been slang for (an) American since the 1960s. Originally used for its risqué meaning, it has become common internet slang for Americans by non-Americans. A particularly popular target is President George W. Bush, whose name, unpopularity and meagre diction complete the joke. (not to mention that “bush” is also slang for female pubic hair!) Usually derogatory, it is also used as an alternative to “USAnian” to distinguish US Americans from Canadians when discussing North American inhabitants.
That’s one definition from Wikipedia. There are, unfortunately, others.