run off at the mouth字面意思是“到嘴边给跑了”（即煮熟的鸭子飞了），这个俚语可以说是非常形象地描述那种说话“喋喋不休/夸夸其谈、说话没完没了、滔滔不绝”的人。

【例句】
Ms. Rosario was running off at the mouth again about the kids playing basketball in her backyard.
罗萨里奥太太又在喋喋不休地说孩子在她家后院打篮球的事了。

He just likes to run off at the mouth. Nobody listens to all that talk.
他就忽悠别人能行，你没看大家都不喜欢听他说话么。