Japanese Idiom Lesson:

口が重い
kuchi ga omoi
This is used when someone speaks only a little or is very quiet.


The antonym of this expression is 口が軽い。 kuchi ga karui. One’s mouth is light.


This idiom literally means, “mouth is heavy.” A heavy mouth doesn’t say much.


佐藤さんは、口が重いので、
デート中なにも話しませんでした。  

Because Sato is naturally quiet, she didn’t say anything during her date.

佐藤 satou—Sato (a Japanese last name)
wa—[topic particle] (written with hiragana “ha” but pronounced “wa” when used as particle.
ので node—therefore; because
デート中 de-to chuu—during a date
なにも nanimo—nothing; not at all
話しませんでした hanashimasen deshita—didn’t speak