Pop Goes the Weasel.
Where do we get the expression "Pop goes the Weasel"?
When you sing:
'That's the way the money goes;
Pop goes the weasel. . . "
you're referring to the opening and shutting of a pocketbook. "Weasel" or "weaselskin" was a popular slang name for a "pocketbook" when the verse was written.
(Why Do We Say It? pg 192)