The Brief: A subtweet is a passive aggressive tweet.

Bluesquiggle
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Subtweets call out a specific person or group on Twitter without directly revealing their identity. If the subject of a subtweet is a public figure, then context will help distinguish identity, though this form of tweeting is typically targeted toward a more personal relationship. Subtweeting can be a form of venting negative emotions without directly engaging with the source of the conflict; however, most users seem to agree that subtweeting is petty.