Poe’s law

Etymology

The law is named for Nathan Poe, who stated a version of it in 2005 in an online forum debate about creationism, observing that a parody of fundamentalism would be indistinguishable from the real thing without an explicit wink. It generalized quickly to all extreme views online. It belongs to a family of tongue-in-cheek internet "laws," named after the person who first articulated them.

noun phrase · /POHZ law/
The principle that without a clear signal of intent, a sincere extreme view and a parody of it become impossible to reliably tell apart.
What it names
It explains why online you genuinely cannot tell whether a wild post is a true believer or a joke — and why the joke is so often mistaken for the real thing, and vice versa.
Example
The account was so over-the-top that Poe’s law kicked in — half the replies took it as satire, half as a manifesto.
Related idea
Poe’s law is about the indistinguishability of sincerity and parody; sealioning is a related bad-faith tactic where someone feigns polite sincerity to exhaust an opponent, weaponizing that same ambiguity.