Prepossession is a prejudice or a preconceived idea about something. You might be accused of prepossession if you decided you were going to dislike your new job before you'd even started working there.
When you've got a strong opinion about a subject — or a person — despite having little information or direct experience, that's prepossession. Your prepossession on the subject of cats might make it hard for you to be enthusiastic about your roommate's new kitten, for example. The obsolete verb prepossess originally meant "to get possession of beforehand." By the 1630's, it came to mean "to possess a person beforehand with a feeling or idea," usually in a positive sense.