A: Hey, I saw my boss smiling from ear to ear. Do you know why?
B: He became a grandfather for the first time yesterday.
A: Oh, really? I found that a bit surprising. I thought he was only interested in work.
B: He certainly looks like a business-oriented person, but I heard that he really cherishes his family very much.
In this situation, what should you say ‘smiling from ear to ear’ in Japanese?
Business
A：上司が満面の笑みを浮かべてたんですが、何があったか知ってますか？
Note: This is not a word-for-word translation.
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Google Translate suggests the same translation.
In English, I think that ‘from ear to ear’ indicates that someone’s mouth is open wide, almost from one ear to the other. I can easily imagine someone smiling widely and showing their teeth — they’re happy!
In Japan, we use expressions like the following,
満面の笑み
喜色満面
I find that we imagine all parts of a face when we try to express someone with a big smile. I mean, both of the former examples indicate the whole face not just the mouth, or not just other individual features, such as the eyes or eyebrows.
満面 means ‘entire face’, so 満面の笑み means smiling using all parts of the face.
喜色 means looking happy, so 喜色満面 means that someone’s entire face conveys happiness.
Kotoba no Hibiki 