Underhappy refers to a mood that is not as happy as one ought or would like to be. The phrase is coined to describe an in-between state between being happy and unhappy.

“高兴未满”指的不是很高兴的一种欠佳的情绪状态，是介于高兴和不高兴之间的情绪状态。

Example:

Many Singaporeans are “under happy” at work, or so says a recently released survey.

一份新近发布的调查显示，许多新加坡人在工作时间都不是太高兴。