Moop has become a way to say "Hi" or "Hello" in the game World of Warcraft. It is used to say "Hello" in guild chat but the most common use is as a General Chat salute in high level raid instances, when a player wishes to make his/her presence known to all other players who are zoned into the same instance.
Repsonses may vary from "Hey Freddie " to "STFU you nab we are raiding here" which often leads to a spate of general chat spam and abuse.
person2: Hey m8 how a u?
person3: Oh no not he again. Who let him in here?
person4: FFS SHUT UP &%#% NOOBS!!!11