I disagree a little with John. Yes, it can be a judgement a person makes about a another person or group, but this is just one context. 'Prejudice' as a verb can be much broader in meaning ... it all depends on context. It is one of those words that media and politics commonly use in one usually negative context, but in fact, there are many other ways less negative where this word can be used.

Margarett is correct. It means to not let others affect your thinking, view or beliefs about someone or something. Prejudiced is not so much an unreasonable opinion about someone (although that is true), it is a judgement a person makes towards a person or group. A person can be prejudiced against tall or short people, rich or poor people, Asians, Americans, Christians, Muslim's, overweight people, etc... In other words, don't let your view or thinking be affected about someone or some group because of what other people are saying. (:

to impair, maybe...

It essentially means 'to influence'. "Don't let others prejudice your thinking" = " Don't let others influence your thinking".