An act or topic that is considered socially unacceptable and is often difficult to even discuss.
Anime often touches on darker subjects widely viewed as "Taboo" these extremes of the dark side of humanity drive a variety of series more sadistic subject matter. The usual culprit being violence, the latter being sex, and sometimes a combination of both. This is not always done simply for "shock value" and in many cases is done to address something in a serious fashion, showing consequences for the actions. Though the usual culprit of Taboo subject matter is the hentai sub-genre. What is considered to be Taboo greatly varies from culture to culture. Cannibalism is universally viewed as Taboo outside some primitive tribes for example of a true extreme, while some cultures find even something as innocent as public displays of affection such as kissing or even holding hands taboo. From an anthropological standpoint this can be a tricky concept, it is also the main drive from region to region on how anime is edited and regionally edited/censored if at all.
The Transformers manga character Legion for example is viewed as unacceptable for release in the US continuity due to the fact having a character in what is primarily a children's franchise who looks like him would be Taboo as he is more appropriate for a mature franchise, on the reverse side, he appears readily in children's manga in Japan.