Coming with any number of 'a's between two and... no real limit - only your lung capacity - 'Gaaaar' was a term coined by pirates with little or no rum left in the bottle.
Designed to vent frustration, the word 'Gaaar' represents a healthy outlet of anger and can, in some cases, prevent the user from going ape-shit.
Modern day users extend beyond just the pirate - anarchists, strange people and others use it, along with it being used as the basis of an entire language in a few small mountain-based communities. 'Gaaaar' is also a great tool to use if you're a zombie - learned from such movies as 'Dawn Of The Dead' (you just know the guy with no arms is giving a good 'GAAAAAAR!'