noun
Refers to the chest area, often used in contexts of clothing, body, or physical sensation.
胸先に冷たい風を感じた。
I felt a cold wind on my chest.
そのドレスは胸先が開いている。
That dress is open at the chest.
胸 is the general word for chest; 胸先 specifically emphasizes the front or surface area of the chest.
Compound of 胸 (muna, 'chest') + 先 (saki, 'tip, front'). The reading むなさき reflects rendaku voicing of さき.