one's mind; one's inner feelings; how someone feels about it
Idiomatic expression referring to a person's true thoughts or feelings, often used when a decision or outcome depends on someone's personal sentiment. Literally 'three sun in front of the chest'.
The final decision depends on what he really thinks.
It's hard to read what the other person is truly feeling.
Compound of 胸先 (むなさき, 'the area in front of the chest') and 三寸 (さんずん, 'three sun', a traditional unit of length). The expression figuratively suggests that a person's true feelings reside just a short distance inside the chest, and decisions hinge on that inner state.