After being mocked by netizens, he immediately fired back on Weibo.
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Internet slang. The character 怼 had been obscure in everyday speech, but exploded in popularity around 2017 to mean 'shoot down / push back'. 回怼 specifically means to push back at someone who attacked you first.
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Standard prescriptive dictionaries list the pinyin as duì, but you may also see duǐ in older sources — modern Mandarin overwhelmingly uses duì for this internet sense.