The unannounced visit felt like an unwelcome intrusion into their confidential meeting.

intrusion 是名词，由动词 intrude（闯入、侵入）加上名词后缀 -ion 构成。intrude 本身由前缀 in-（表示"进入"）和词根 trudere（拉丁语，意为"推、挤"）组成。intrusion 指"未经允许的进入或干涉"，带有负面含义。