Both that phrase and the accent that goes with it are strictly Hollywood. The pirate phrase "Arrrgh" appeared in film as early as 1934, by Lionel Barrymore, and Robert Newton the actor who played Long John Silver in the movies and on TV through much of the 1950s. Newton was from Dorset, and the regional accent he brought to the movies included a rolled "r." Though Dorset may well have produced its share of sailors, they were hardly the only pirates out there; many seamen*—and especially the outlaws on pirate vessels—were people who struck out from oppressed nations, like Scotland and Ireland, to start over on the high seas.

Well, the use of the word "Arrrgh" was likely just a movie affectation, from as early as 1934. The accent and syntax was popularized by the 1950's movies about Long John Silver. HOWEVER, there IS a history behind the vocabulary and syntax of the sailors of yesteryear, even pirates. "Mate" and "matey", for example, are real terms. A "mate" is slang, short for "shipmate". And "matey" is yet another slang term for the same. The First Mate was actually a specific person on the crew: an officer immediately below the Captain in rank. Underclass British sailors drove the dialect(among the pirates who originated from the British) along with a smattering of French Italian, Spanish, and Dutch. Words and phrases like "land lubbers", "shiver me timbers", "scuttlebutt", "poop deck", "avast", "grog", and more are real and have their origins in one or more of several languages. The life of a sailor was harsh, especially in the days of wooden ships and iron men. And many of the crewmen were anything but well educated and well brought up people. In fact, some were unwitting crewmen by virtue of "press gangs"...they would go out for a night of drinking, only to wake from a drunken stupor the next day to find out they "signed on" for a period of service on some ship or other. So the actual lingo spoken was not only a mix of nautical terms from many different countries, but also influenced by people who had no formal education as well. And a lot of those people, by the very nature of where they came from and the backgrounds commonly associated with them, were NOT goody-two-shoes. But "Arrrrgh" and "walking the plank" are a couple "pirate" phrases that don't seem to be grounded in fact...just movie embellishments!