Japanese Idiom Lesson:

口くちが重おもい
This is used when someone speaks only a little or is very quiet.

The antonym of this expression is 口くちが軽かるい。 One’s mouth is light.

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

佐藤さとうさんは、口くちが重おもいので、デート中ちゅうなにも話はなしませんでした。  
Because Sato is naturally quiet, she didn’t say anything during her date.

佐藤さとう—Sato (a Japanese last name)
は—[topic particle] (written with hiragana は but pronounced わ when used as a particle.)
ので—therefore; because
デート中ちゅう—during a date
なにも—nothing; not at all
話はなしませんでした—didn’t speak