Ahoy = Hello (in Dutch)
Hoy = A small sloop-rigged coasting ship or a heavy barge used for freight
In the 1870's, inventor and Scottish born Alexander Graham Bell invented the telephone. Bell's preferred salutation, Ahoy-hoy was derived from the nautical term above.
It really gained popularity when Mr. Burns used it on the Simpsons a few seasons ago, but the internet has adapted it for use as a common greeting. It's commonly spelled Ahoi hoi online.
It is that simple, you should try it some time.
Monk 2: Why I'll be Svendened! A ripe young lad outside the walls! Ahoy-hoy!