On Wikipedia, "coatracking" is an original metaphor understood to mean a Wikipedia article that isn't really about the article as it purports to be, but instead is written to make a tangential point, sometimes an attack on the subject itself. In this metaphor, the nominal subject is the coat-rack itself, and the arguments it supports are the coats hung upon it.
Determining what is and is not a true "coatrack" is a complicated matter. What is irrelevant, biased, or unfair? The answer is different in every case, and so the "Coatrack articles" essay contains numerous hypothetical examples named for common refrains, like "But it's true!" and with amusing sub-titles like "Some Famous Dude Did It so It Must Be Good".
Identifying a coatrack is closely related to the topic of "due and undue weight", which we'll explore in-depth next week.