An animal seen from below. Its back is covered — scales, fur, shell — but its belly faces the ground, pale and bare. Two things are true of that surface at once: you do not normally see it, and nothing is protecting it. Every use of underbelly leans on one of those two facts, and the strongest uses lean on both.
可以想成從下面看一隻動物。牠的背上有鱗、有毛、有殼。肚子卻朝著地面，顏色偏淡，什麼都沒有。這一面同時有兩個特點。第一，平常看不到。第二，完全沒有保護。underbelly 的每一種用法，都靠著其中一個特點；最有力的那幾種，兩個一起用上。
From the word’s history
Wiktionary gives the formation as under- + belly. The word is built, not borrowed, so the image is already in the parts.
Wiktionary 記載這個字就是 under-（下面）加 belly（肚子）組成的。它不是外來字，而是英文自己拼出來的，所以畫面本來就寫在字裡：動物翻過來才看得到的那一面。
Wiktionary, "underbelly"
登入就可以看每一支怎麼連回核心畫面。