The secret service does not explain things like this. Some say that Tumbler is not the right code name. Some the code was given to him when his father was President and that it was similar to his father's code of Timberwolf and that code names are not selected for what they mean but for their sound. One person says that Tumbler is a character in Captain America comics . One person says that tumbler is a drinking glass and seemed appropriate for "W". Some think it is short for Texas Tumbleweed.
The Secret Service code names for individuals are chosen randomly and do not have any significant meaning. Tumbler was likely selected as George W. Bush's code name simply as a random and unique identifier assigned to him by the Secret Service.
Washington
Blue
to be president.
Barnie
chuck e cheese
May and Flower
George Walker Bush
George H.W.Bush
Barbara Pierce Bush
working at a public official
70 zillion years
January 20, 2001