The etymology is useful here, and comes from Latin phrases meaning pre- and 'judge'. I think the word etymology really helps here, in a sense it has all the sense of assessing someone before encountering them. You have already formed a complete system of thought (usually negative) that you ascribe to an individual or group of individuals.
This prejudice has an unintended side effect - it's very difficult to rid one's self of the prejudice even when meeting with a stigmatised individual. Thus, no matter what the other individual says or does, they cannot fully extricate themselves from the prejudice of the other, nor is it their job to do so.