口くちを挟はさむ

expression, godan verb (-mu)

cut into a conversation; interject; butt in

Used when someone joins a conversation uninvited, often with an opinion or comment. Can be neutral or slightly negative depending on context.

• 会かい議ぎ中ちゅうに彼かれが口くちを挟はさんだ。

  He cut into the conversation during the meeting.

• 二人ふたりの話はなしに口くちを挟はさまないでください。

  Please don't butt in on their conversation.