Picture a wall with nothing behind it: low ground, open on three sides, every path leading straight in. You can put people on that wall, but it will not hold. That is the picture the word carries, and it works just as well when the thing being held is not a place but an argument. Someone asks you to account for what was done, you start to speak, and there is no line that holds.
From the word’s history
Simply in- plus defensible: not able to be held. The packet's etymology tree shows the same negative prefix that English took from Latin through Middle English, added to the ordinary word for something that can be defended.
就是 in-（不）加上 defensible（守得住的、辯護得了的）。這個否定字首是從拉丁文經中古英語進到英文的。所以整個字字面上只說一件事：守不住。
Wiktionary etymology tree supplied in this packet (source_ref kaikki:3f6f69a190036ae4:1217847407): in- + defensible
登入就可以看每一支怎麼連回核心畫面。