My parents were rather annoyed that I hadn't told them about it.

annoyed 是动词 annoy 的过去分词形式，在这里作形容词使用。annoy 的意思是"使烦恼、使恼怒"，所以 annoyed 表示"感到恼火的、生气的"。 在句中 annoyed 作表语，描述主语 my parents 的情绪状态。