From Wiktionary, the free dictionary
Misconstruction of bagatelle (“trifle”).
bag of shells (plural bags of shells)
- Used other than figuratively or idiomatically: see bag, shell.
- A trifle.
1914, Matthew M. Colton, Frank Armstrong at college[1], page 39:For an ordinary intellect, yes, but for me a mere bagatelle, or bag-of-shells, as the ancients have it.