Short for 'All
your base are belong to us', meaning something like 'we own you' or 'you got owned' or something. anyway. The popularity of this phrase is unexplainable but it has circulated the net for years in pictures and text. Originally comes from a game for the
sega megadrive system called
zero wing, a space shoot-em-up released in japan in the '80s. The game was translated into english (very poorly at that, it almost qualifies as engrish) and released in europe in 1989. For more details on how this phrase came to be, google for 'AYBABTU'.