The child was called a potty-mouth by the teacher for swearing at school.

potty 原指"小马桶"，在俚语中引申为"愚蠢的""微不足道的"。 mouth 本义是"嘴巴"，这里指"说话的方式"。 potty-mouth 是俚语，字面意思是"马桶嘴"，用来形容"经常说脏话/粗俗话的人"。