Sea-lioning is a style of personal interaction in which one person – the "sea-lion" – insists on engaging one or more other people on a subject they wish to abandon, and/or requiring explanations or defenses of their position on that subject. It is most notably characterized by an unfailing adherence to the appearance of civility while in fact being highly disruptive and obnoxious, and seizing upon even the tiniest lapses in civility by others as evidence that they are in the wrong (aka tone-policing).
It works by way of the fact that there is always a limit to someone else's knowledge and ability to defend their position, but no limit on the ability to ask "why?" to any given statement.
It is related to the just asking questions debate methodology, and is arguably a form of rhetorical deception in that it exerts influence without justification by shifting the window of discussion (see interpretive framing).
The term "sealioning" appears to have originated with Wondermark comic #1062: The Terrible Sea Lion (2014-09-19).