bringdown
bringdown {n.}, {slang}, {informal}
1. (from "bring down", past "brought down"). A critical or cutting remark said sarcastically in order to deflate a braggard's ego.
John always utters the right bringdown when he encounters a braggard.
2. A person who depresses and saddens others by being a chronic complainer.
John is a regular bringdown.
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Source: A Dictionary of American Idioms