一个愁字

Direct translation is 'one word of worry/sadness'. It implies that there's much to be sorrowful or concerned about but only a single character ('忧') encompasses it all, leaving many unspeakable sentiments hidden inside. It hints the owner may possess inner worries, sorrows or stresses but tries to keep composed on the surface. The Chinese culture places high value on reserve and dignity, thus it reflects someone with rich internal emotions yet remaining stoic.