bet one's bottom dollar meaning

[American idiom]
to be quite certain (about something). (Both are informal and folksy. A bottom dollar is the last dollar.)
• I'll be there. You bet your bottom dollar.
• I bet my bottom dollar you can't swim across the pool.
• You bet your life I can't swim that far.
• I bet my life on it.

[American slang]
Fig. to be quite certain (about something). (A bottom dollar is the last dollar.)
  I'll be there. You bet your bottom dollar.
  I bet my bottom dollar you can't swim across the pool.
  You bet your life I can't swim that far.

• bet one's bottom dollar on:     (informal) & ...
• i'll bet you my bottom dollar:    [American slang]in ...
• bottom dollar:    [American slang]n. ...