Picture a lock that has gone dead. The bolt is all the way out, the key will not turn, and the door will neither open nor close. Nothing is broken and nothing is missing — the parts are simply pressed against each other so that nothing can move. Every use of this word is that same stuck bolt, borrowed for people, for talks, or for machines.
The word is simply dead plus lock. English uses dead in this way in several places — a dead stop, a dead end — to mean that something has no give left in it at all.
這個字就是 dead 加 lock。英文常這樣用 dead 表示「完全沒有轉圜」，像 dead stop、dead end，這裡的 dead 也是同一個意思。
Wiktionary etymology supplied in this evidence packet (kaikki:3f6f69a190036ae4): "From dead + lock."
Three branches
真正的那把鎖
這一支就是畫面本身，沒有比喻：一根不會自己彈回來的鎖舌。
noun · C1
A strong door lock whose bolt slides deep into the door frame and can only be moved with a key, so it cannot be pushed back by a card or a hand. The source marks this as British English; the same lock is called a deadbolt.
防盜門鎖（死栓鎖）
人跟人之間頂住不動
把卡死的鎖舌換成兩群人：兩邊一樣強、都不肯讓，於是整件事停在原地。這是一個想像上的連結，方便理解，不是說這個意思是從門鎖「生」出來的。
noun · B2
A situation in which two sides in an argument or a set of talks both refuse to move, so nothing can be settled and nothing goes forward.
僵局
機器之間互相等待
同樣是兩邊互相頂住，只是換成程式；巧的是電腦領域真的把那個資源叫作 lock，畫面和用詞就完全對上了。
noun · C1
In computing, a situation where two programs or two devices each stop and wait for the other to answer first, so neither one ever starts again and the work stops.
死結（程式卡住）