1 在生活中，我们有时会碰到话多的小朋友，就可以称其为

a mouthy child

2 bad mouth sb 指说坏话。（mouth在此是动词词性）

Do not bad mouth me.
别说我坏话。

3 down in the mouth

Down in the mouth是口语，意思是“沮丧、垂头丧气”。
例如：
Why do you look so down in the mouth ?（你为什么显得这样消沉?）She gets down in the mouth over the least little thing.（她动不动就情绪低落。）

4 word of mouth/by word of mouth

Word of mouth就是我们常说的“口碑”。By word of mouth是指人们经由一传十，十传百的口传方式得知某件事，也就是藉着口耳相传，大家告诉大家的意思。
例如：
A：This restaurant is so small, but it's always crowded. How did you find out about it?　这家餐厅很小，但却始终客满。你是怎么知道这家餐厅的？
B：I heard by word of mouth that they had great food.我是藉着人家口耳相传，知道他们卖的东西很好吃的。

5 Take the words right out of one’s mouth意思是“说出了某人想说的话，观点不谋而合”。
例如：
A: Did you see that girl in our English class. She is really beautiful. 你看见英语课上那个女孩儿了吗？她可真漂亮。
B: You took the words right out of my mouth.我也正想这么说。