A/V Club etymologizes the word 'jawn'

in a brief interview that also explains philly favorites 'hoagie,' 'down shore,' and 'yo':

There was a suggestion that it could have been some sort of variation on “John,” the name, but people mostly talked about it as coming from “joint,” which seems like the most obvious etymology to me. “Joint,” meaning marijuana cigarette, got extended into referring to anything that’s fun or pleasurable, as in, “That’s the ______.” But it’s difficult when there aren’t a lot of good sources, so there’s a lot of speculation. 

via miz jones