@keearis@hellsite.site serious answer: It's a term that came from "Subtweet", where you tweet about someone who may or may not be able to see your public tweet, so you don't @ them or mention them by name, but you're being JUUUST vague enough that only folks who know the person will know you're talking about said target of the subtoot without the target actually knowing, in theory.

Or being less vague so the target DOES know, but nobody else will.

"Subtoot" is the same, but Toot instead of Tweet.