What is a commonly understood meaning of "bummer"?

The term "bummer" is commonly used in informal English to denote a situation that is disappointing or unfortunate. It reflects a sentiment of regret or dissatisfaction about an event or circumstance that didn't turn out as hoped. For instance, if someone planned an outdoor picnic but it rained on that day, they might say, "That's such a bummer." This captures the essence of experiencing something negative or a letdown. The other terms do not accurately reflect the straightforward meaning of "bummer": a pleasant surprise, significant achievement, or joyful event are all positive concepts, which contrast with the inherent negativity associated with a "bummer." Thus, identifying "a misfortune or disappointment" accurately aligns with the common use of the term.