Just learnt this phrase today.
It literally means: the hidden side of the heart, which refers to compassion. Well, compassion is not even a close match to what it means to say. But I think it is the closest word.
This phrase comes from a very beautiful passage from the Mencius (公孫醜上) (The English title is completely different from the Chinese title):
All men have a mind which cannot bear (to see the suffering of) others. … If now men suddenly see a child about to fall into a well, they will without exception experience a feeling of alarm and distress.
From this case we may perceive that he who lacks the feeling of compassion is not a man;
that he who lacks a feeling of shame and dislike is not a man;
and that he who lacks a feeling of modesty and yielding is not a man.
The feeling of compassion is the beginning of human-heartedness (仁).
The feeling of shame and dislike is the beginning of righteousness.
The feeling of modesty and yielding is the beginning of propriety.
The sense of right and wrong is the beginning of wisdom.
Man has these four beginnings, just as he has four limbs.
Since all men have these four beginnings in themselves, let them know how to give them full development and completion. The result will be like fire that begins to burn, or a spring which has begun to vent. Let them have their complete development, and they will suffice to protect all within the four seas. if they are denied that development, they will not suffice even to serve one’s parents.
[Mencius (孟子)， Mencius (公孫醜上), IIa, 6. Translated by James Legge, The Chinese Classics, Vol 2 (Hong Kong: 1861)
N.B.：I have changed the english translation a bit. I prefer to use “compassion” rather than “commiseration” because personally, I think commiseration does not hit the right note.]