Think of a cloth bag. There is one opening at the top, and the bottom is sewn shut. Anything that goes in has to come back out the same way, because there is nothing at the far end. French named this word exactly that: the bottom of a bag. Every use of cul-de-sac can be laid on that picture — a street you must reverse out of, a plan that stops dead, a pouch in the body with one way in.
Borrowed from French cul-de-sac, made of cul (bottom) + de (of) + sac (bag), so the literal sense is the bottom of a bag.
借自法文 cul-de-sac，由 cul（底部）、de（的）、sac（袋子）組成，字面就是「袋子的底」。這個字面義一路撐著今天所有的用法。
Wiktionary entry supplied in this packet (source_ref kaikki:3f6f69a190036ae4:751237178:16916): Borrowed from French cul-de-sac, from cul (bottom) + de (of) + sac (bag, sack).
Two branches
只有一個開口的實體空間
這一群是把袋子的形狀直接搬到看得到、摸得到的東西上：一頭開、一頭封死。
noun · B2
A short street that is closed at one end, so cars can only go in and come back out the same way. In many neighbourhoods the closed end opens into a round space where a car can turn around.
死巷；無尾巷
走不下去的處境
同一個袋子的畫面，換成看不見的路：一個想法、一場談判也可以有「走到底就沒了」的盡頭。
noun · C1
A situation, an argument, or a line of thinking that leads nowhere, so no more progress is possible and you have to go back and try another way.
僵局；死胡同