A damp squib is a disappointing outcome, a mediocre or failed enterprise, an anti-climax. A squib, which the OED maintains is an imitative word, is a small firework or explosive device dating from the early 16th century. Dating from around the same time, it had an alternative meaning for a lampoon or piece of satirical writing but this meaning has largely fallen away. A damp squib would be a firework that fails to ignite or explode properly because of excessive moisture and is therefore a metaphor for a disappointing outcome. This figurative usage is first attested from the mid-19th century.