It's a nickname for George W Bush and comes from the Texas pronunciation of the letter 'W'. His father similarly gets nicknamed 'Poppy'.
W
Jeb Bush
dubya
Dubya for George W. Bush, an exaggeration of Texan pronunciation of the name of the letter 'w', President Bush's middle initial.
George dubya bush
george dubya bush
The current President's father was also President George Bush. This one is George W. Bush, so he is nicknamed "Dubya (W)" to differentiate him from his father. The W is for Walker.
Dubya Tee Eff - 2008 was released on: USA: June 2008 USA: 10 June 2008 (48 Hour Film Project)
You can understand what George "Dubya" Bush is saying.
43rd president George W. Bush (Dubya)
You can understand what George "Dubya" Bush is saying.
One of George W. Bush's nicknames is "W" (pronounced as "Dubya"), to distinguish him from his father. Some call him "Poppy". The Secret Service may have an altogether different slant on this issue.