In Reel Life: Rocky is a lefty. He tells Adrian that he's a southpaw, and explains the term's etymology to her: "You know where southpaw comes from? I'll tell ya. A long time ago there was this guy, maybe a couple a hundred years ago, he was fighting around, I think it was around Philadelphia, and his arm -- he was left-handed -- and his arm was facing toward New Jersey, you see? And that's south. So then naturally they call him south paw. You see? South paw, south Jersey, South Camden, south paw. You know what I mean?"
In Real Life: Rocky's explanation includes a grain of truth. According to the "New Dickson's Baseball Dictionary," the term was coined in the late 1800s to describe left-handed pitchers, who, facing west in most ballparks, had their left arms hanging on the south side of the ballpark. A sportswriter, Harry Grayson, investigated this theory in 1951, and determined that most ballparks did, indeed, place home plate on the west side of the diamond, on the principle that this would keep the sun out of hitters' eyes during day games. The term has been applied to other sports, including boxing, and obviously came into general use. The term "northpaw" never caught on, though.
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