Red herring
Etymology
A herring turns reddish-brown and pungent when smoked, and the figurative sense comes from a tale that such a fish, dragged across a trail, could throw hounds off a scent. The polemicist William Cobbett popularized the image in 1807 to describe distracting the press. Etymologists note the dog-training story is likely his own invention, but the metaphor stuck regardless.
noun phrase · /RED HAIR-ing/
Introducing an irrelevant point to divert attention from the actual issue.
What it names
Catching it keeps a discussion from being quietly steered onto a tangent that settles nothing.
Example
Asked about the budget shortfall, the mayor started talking about how hard his staff works — a red herring.
Related idea
A straw man also derails a debate, but by attacking a distorted claim rather than changing the subject.