KHAAAAAAAAN! In case you are a total noob and do not know where it comes from, it is from Captain Kirk yelling at Khan in Star Trek II: The Wrath of Khan. Video clip here.
*Khan Noonien Singh, commonly shortened to Khan, is a fictional villain in the Star Trek science fiction franchise. The character first appeared in the Star Trek: The Original Series episode "Space Seed" (1967), and was portrayed by Ricardo Montalbán who reprised his role in the 1982 film Star Trek II: The Wrath of Khan. In the 2013 film Star Trek Into Darkness, he is played by Benedict Cumberbatch.
The character once controlled more than a quarter of the Earth during the Eugenics Wars of the 1990s. After being revived from suspended animation in 2267 by the crew of the Starship Enterprise, Khan attempts to capture the starship, but is thwarted by James T. Kirk and exiled on Ceti Alpha V to create a new society with his people. In Star Trek II: The Wrath of Khan, set fifteen years after "Space Seed", Khan escapes his exile and sets out to exact revenge upon Kirk.
**Pizz would yell KHAAAAAAAN! in voice chat every now and then and it was recorded to Toidy's Ipod mistakenly. She was driving down the freeway to work one day seat dancing to her ipod when suddenly very loudly KHAAAAAAAAAAN! came blaring thru her speakers in Pizz's yelling voice scaring the crap out of Toidy in the process. As she related the story later in voice chat it was found to be very amusing by all, except for maybe Toidy.