Subtweeting
Etymology
A blend of "subliminal" (or "sub-," beneath) and "tweet," coined on Twitter in the early 2010s. The "sub" marks the message as operating below the surface — aimed but not addressed. The equivalent on other platforms is sometimes called "vaguebooking."
noun · /SUHB-tweet-ing/
Posting a pointed comment about a specific person without naming or tagging them, so only the in-the-know understand the target.
What it names
A passive-aggressive maneuver that lets someone attack while retaining deniability, and gives the target a word for the uncomfortable sense that a post is about them.
Example
She never named him, but that bitter post about "people who forget their friends" was obviously subtweeting.
Related idea
Vaguebooking is the Facebook cousin — a deliberately cryptic post fishing for concern — whereas subtweeting has a specific, hidden target in mind.