What does the phrase "just a bagel" mean in Chicago (the musical)?
Solution 1:
The word "doughnut" is occasionally used to mean "zero"; see this article for an example.
Obviously, this meaning originated in the similarity of shape.
A bagel, like a doughnut, has a hole in it, so I expect the songwriters intended the meaning to be the same. I also expect they thought this meaning would be clearer than it actually seems to be.