Final answer:
The claim that the second lady's secret service code name was "dollar bill" is probably a humorous remark, not a factual statement. Secret Service code names are classified and not publicly disclosed.
Step-by-step explanation:
The joke that the second lady's secret service code name became "dollar bill" in Laura Bush's White House Correspondence after-dinner speech is likely a humorous remark rather than a statement of fact. The roles and public profiles of political figures like the first lady or the second lady are often characterized with such humor to lighten the atmosphere at official events.
While the first ladies and other White House officials do sometimes receive code names from the Secret Service for security purposes, the exact code names are not typically shared with the public and are considered classified information. Therefore, the assertion in the question may be better understood as a playful jest rather than a true disclosure of classified information.