In Halo 3 what does IWHBYD stand for?

In Halo 3, IWHBYD is an acronym for "I Would Have Been Your Daddy".

The Halo 3 IWHBYD Skull is located on the level The Covenant. This skull has no actual gameplay effects, but unlocks extra dialogue for humorous purposes.

The name of the skull is based on a statement, uttered by Sergeant Johnson in Halo: Combat Evolved (in the last level on Legendary difficulty): "I would have been your daddy, but a dog beat me over the fence!" It is also the name of the first chapter of the campaign level Assault on the Control Room in Halo: Combat Evolved. Not only is the skull's name a reference to this line, but the skull's description reads "But a dog beat me over the fence".

http://halo.wikia.com/wiki/IWHBYD_Skull_%28Halo_3%29