Don't make such negative assumptions about your coworker when you don't even know him.

assumptions 是名词 assumption 的复数形式，assumption 来自动词 assume（假设、假定），指"未经证实的推测或看法"。 negative 是形容词，意思是"消极的、负面的"，用来修饰 assumptions，表示"消极的假设"。