A pejorative is a word or phrase that is used to express contempt.
'Idiot' 'plonker' and 'moron' are pejoratives. 'Chav', 'Tree-hugger' and 'Bottom feeder' are other recent examples.
In the classroom
Learners are likely to hear this language informally in English speaking contexts or through exposure to authentic language in films. It may not be appropriate to teach students pejoratives and other slang, and many teachers will choose to teach it for recognition only. Context is important, e.g. a learner working in the UK may need to understand a lot of British slang.