Is 金口难开 an idiom, and if so what would be a suitable CC-CEDICT definition?
金口難開
..... 後來，便有了「金口難開」這個成語，表示：嘴巴像金子一樣珍貴，不輕易張口用來說話；形容一個人沉默寡言、不愛說話。

Base on the story, 金口難開  originally describe &quot;someone value his own words like gold, do not open his mouth to speak unless it is necessary&quot; ; Now it is used to describe &quot;someone being reticent; a man of few words&quot;

金口 = golden mouth (valuable speech)

難開 = 難得打開 (rarely open)