He showed opportunism in the negotiation by exploiting the other party's weaknesses for personal gain.

opportunism 是由名词 opportunity（机会）加上后缀 -ism（表示主义、学说）构成的抽象名词，字面意思是"机会主义"，指为了个人利益而利用一切机会的行为方式，通常带有贬义。