Other forms: prepossessingly
If someone's prepossessing, they make a great first impression. You may be dreading the first day of school, until you meet your prepossessing new teacher.
People and things can be prepossessing because they're so attractive, charismatic, or otherwise immediately appealing. Your friend's huge, fancy new house may seem quite prepossessing, for example, while your dog is anything but prepossessing after rolling in the mud. In fact, you might call him unprepossessing, which is much more commonly used than its antonym. Prepossessing is from the verb prepossess, "to possess a person beforehand with a feeling or idea."