Subtweeting is rude. Luckily for us, it may be dead. Buzzfeed has declared it so.
I'm going to share the Urban Dictionary definition anyway:
Ugh. Wasn't there enough middle school behavior on the Internet? The important elements of subtweeting are: 1) it must be mean 2) it must not mention the person or people by name and 3) the unnamed person or people must be able and likely to see it. Complaining about unnamed celebrities or politicians, people who can not see the Twitter feed or are strangers and unlikely to, or naming or mentioning a user, do not make the 140-missive a subtweet.Indirectly tweeting something about someone without mentioning their name. Even though their name is not mentioned, it is clear who the person tweeting is referring to.
Making the subtweet more complicated: many feel that the tweet must be both funny and not passive aggressive to others. That is, while you and the unnamed recipient understand that he, she, or them, are the source of the tweet, unrelated readers do not know the person or that someone has inspired humorous ire.
Personally, I have a hard time seeing humor in any subtweet. I am loathe to credit Buzzfeed, but I think this is a good rule for Internet behavior (if you're not going to be nice in the first place): "The new rule: If you want anyone to pay attention to your beef, name names." We're all adults here, let's all agree to have the spine to actually call people out when we call them out.
Subtweeting, Urban Dictionary
How subtweets are ruining Twitter, Slate
The Subtweet Is Dead, Buzzfeed
The Internet Talks To The Internet About The Internet, Makes Fun Of You, The Awl
The Art of Subtweeting, Social Chic