Today’s topic is idioms related to the word “mouth”.
今日（きょう）のトピック は、口（くち）に関（かん）する慣用句（かんようく）です。
口がかたい
It literally means “one’s mouth is hard/firm” but it is used to mean that “one can keep secrets; trustworthy”.
口が軽（かる）い
かるい means “light” so if someone has a light mouth, it means that he has a loose tongue and can’t keep secrets!
Even though the opposite of かたい(hard) is やわらかい(soft), we would not say 口がやわらかい to refer to someone who has a loose tongue. Instead we say 口がかるい.
口が重（おも）い
This phrase doesn’t mean “one can keep secrets”. It rather describe someone “being reluctant to speak”. So 口が重い人 means someone who’s not very talkative and on a quieter side.
悪口（わるぐち）を 言（い）う
悪口（わるぐち）means “bad mouth”. So this phrase means “to badmouth; talk behind someone’s back”.
お口直（くちなお）し
It means “palate cleanser”.
ex) お口直（くちなお）しに、フルーツは いかがですか？
How about some fruits as a palate cleanser?
一口（ひとくち）
ex) おいしそう！一口（ひとくち）、ちょうだい！
Looks yummy! Let me have a bite!
口コミ
It means “word of mouth”
ex) この本（ほん）は、口コミで とてもよく 売（う）れている。
This book is selling very well by word of mouth.
Of course, there are a lot more expressions using the word 口（くち）but I just introduced a few of the most important ones here.
I hope this post was helpful. Let me know if you have any question.
口を割（わ）る
It means “to confess”.
わる is a u-verb meaning to divide; split; break something. For example:
泥棒（どろぼう）は、窓（まど）ガラスを 割（わ）って 家（いえ）に入（はい）った。
The burglar broke the window to break into the house.
Of course, when your mouth is broken, you are forced to confess your secrets. This expression is usually used in rather serious situations. For example, in a mystery novel, you may find a sentence like:
あの犯人（はんにん）は、どうやって殺（ころ）したか、やっと口を割（わ）った。
Finally, the criminal confessed how he killed him.
However, it’s also possible to use this phrase jokingly between friends.
For example, a girl is asking her friend about her hot date and say:
きのう、誰（だれ）とデートしたの？口を割（わ）りなさいよ。
Who did you go out with last night? Confess!
By the way, the ending なさい is added to a verb-stem (the stem of masu-form) to turn a verb into imperative form.
口が悪（わる）い
It means to “have a sharp tongue”.
悪口（わるぐち）を 言（い）う
悪口（わるぐち）means “bad mouth”. So this phrase means “to badmouth; talk behind someone’s back”.
お口直（くちなお）し
It means “palate cleanser”.
ex) お口直（くちなお）しに、フルーツは いかがですか？
How about some fruits as a palate cleanser?
一口（ひとくち）
ex) おいしそう！一口（ひとくち）、ちょうだい！
Looks yummy! Let me have a bite!
口コミ
It means “word of mouth”
ex) この本（ほん）は、口コミで とてもよく 売（う）れている。
This book is selling very well by word of mouth.
Of course, there are a lot more expressions using the word 口（くち）but I just introduced a few of the most important ones here.
I hope this post was helpful. Let me know if you have any question.