Lunchbag letdown

[Mary Mallozziberg]

A [lunchbag letdown] occurs when something you had been looking forward to turns out to be far worse than originally anticipated.

The term originates in the lunchroom (be it school or office or otherwise) where after a long [hard day] someone sits down with their [lunchbag] hoping for something tasty. Instead they find something unappetizing, whether it be the wrong lunch, its moldy, its cold, or any other multitude of problems.

After such a situation, it can be said that your lunchbag was indeed a let down.