In language learning, taboo language or subjects are areas that are regarded as prohibited by the culture of the learners or the teacher, therefore inappropriate for the classroom.

Example
Some learners in the Middle East regard the subject of same sex relationships as a taboo subject.

In the classroom
Taboo subjects, if carefully handled, can be a useful way to stimulate a response from learners, e.g. in a discussion lesson. It may be important to discuss taboos if learners are studying in a country where they need to understand the target culture.