Joke, as relayed by Harry Golden, in the Jewish Daily Forward. A Hassid Jew from New York City had reason to travel to the American South, somewhen in the early 20th century. Being a(n?) Hassid, he wore a long black coat, uncut sideburns, and a big black hat. Not being able to find kosher food, he settled on a diet of boiled eggs, which (being somewhat not used to American usages) he carried, shucked and ate at will, discarding the shells along the road.
At which point, he encountered a local, who regarded him with some wonder, not unmixed with fear, and speculated upon the man's sanity.
At which point, the Hassid (feeling himself a True American) exclaimed "Ain't you ever seen a Yankee before?"
This has become (in some circles) a shorthand for "what are you looking at?"