Anything that doesn’t work properly, or fails to come up to expectations; a dud.
A firework that has been wet and therefore fails to go off correctly.
A feeling of disappointment, akin to depression, arising from the realization that something is not what it was expected or believed to be, possibly accompanied by philosophical angst from having one's beliefs challenged.
The act of freeing from an illusion; the state of being freed therefrom.