This door is confoundedly hard to open; I've tried several times without success.

confoundedly 是副词，由动词 confound（使困惑、挫败）加上后缀 -ed（过去分词形式）和副词后缀 -ly 构成。字面意思是"令人困惑地、令人恼火地"，在口语中常用来加强语气，相当于"该死地、讨厌地"，表达说话人的挫败或恼怒情绪。