chest

Chests are containers, generally used in the home, to hold things that are considered precious, or while travelling, to hold necessities, all the traveler has or needs. They are large and sturdy; they have dignity; and despite existing for what is within them, they often have their own considerable value, both monetary and sentimental. Much of this is also true for the word's anatomical referent, though most of that truth lies in the heart's metaphorical contents. A body's chest is sturdy, strong; it is a prominent body part in discourse of dignity; whether its purpose is its physical or figurative contents is often debated, but those contents are precious, and all we have or need.

We speak of "getting something off one's chest" because of the importance of these contents; when they are tainted by difficult or unhealthy feelings we are burdened by the presence, as such contents are not worthy of such an eminent receptacle.