After losing his job, he turned into a paranoiac, constantly believing his coworkers were talking behind his back.

paranoiac 是名词，由形容词 paranoid（偏执的）加上后缀 -ac 构成，指"偏执狂患者"或"有偏执倾向的人"。这个词源于希腊语，para-表示"偏离"，noia表示"思维"，合起来指"思维偏离正常状态的人"。