His constant interruptions were a major source of irritation.

irritation 是名词，由动词 irritate（激怒、使烦躁）加上名词后缀 -ion 构成。irritate 本身来自拉丁语 irritare（刺激、激怒），后缀 -ion 表示动作或状态。irritation 指"恼怒、烦躁的状态或感觉"。