1640s, "causing prejudice," present participle adjective from prepossess. Opposite meaning "causing agreeable first impression" first recorded 1805.
1610s, "to get possession of beforehand," from pre- + possess. Meaning "to possess (a person) beforehand with a feeling, notion, etc." is from 1630s; specifically, "to cause (someone) to have a favorable opinion of something" (1640s). Related: Prepossessed; prepossessing.