Misconstruction of bagatelle (“trifle”).
bag of shells (plural bags of shells)
- Used other than with a figurative or idiomatic meaning: see bag, shell.
- A trifle.
1914, Matthew M. Colton, Frank Armstrong at college, page 39:
For an ordinary intellect, yes, but for me a mere bagatelle, or bag-of-shells, as the ancients have it.