One way to hold this word: a thing still has its shape, but nothing inside it is holding the weight any more. The wall stands, yet the joints have . The plan is still written out, yet the evidence under it is gone. The person is still going about the day, yet the body or the mind is not carrying them. Unsound names that gap between the shape you can see and the strength that should be inside it.
From the word’s history
The source gives the word's make-up directly: un- + sound. Sound on its own is the old word for whole, firm and healthy, and un- takes that away, which is why one word can cover walls, arguments and bodies.
來源直接給了組成：un- 加上 sound。sound 這個字本身的意思是「完好、結實、健康」，un- 把它拿掉。所以同一個字才能同時講牆、講說法、也講身體——被拿掉的都是那個「完好」。
Wiktionary (kaikki extract, source_ref kaikki:3f6f69a190036ae4:672177704:7522): "From un- + sound."
登入就可以看每一支怎麼連回核心畫面。