A 19th century euphemism for homosexuality. It was "invented" by Oxford and Cambridge Classics professors.

Every text dealing with male homosexuality was labelled as dealing with The Unspeakable Vice of the Greeks, which meant that it was not to be discussed in classes, or in serious scientific discourse (as the universities certainly did not want to encourage such perverse behaviour, and the very discussion of such things was abominable in their eyes).

Naturally, when a professor labelled a text (or a section of a text) as dealing with The Unspeakable Vice of the Greeks (effectively forbidding the reading or discussion of it), this text became the most popular reading material after-class.

This policy of censorship is regarded today as more than a little ridiculous, as it left huge holes in many texts (most notably Plato's Symposium) that were not to be discussed or researched.