The uncle had warned her so that she wouldn't interfere with what happened at the inn, but it had the opposite effect on Mary.

opposite 是形容词，由前缀 ob-（表示"反对、对抗"）和词根 pos（表示"放置"）加上形容词后缀 -ite 组成，字面意思是"放在对面的"，引申为"相反的、对立的"。 在句中指叔叔的警告不仅没有阻止玛丽，反而产生了与她叔叔期望完全相反的效果。