沒有人看守
這一家就是畫面本身：真的有一道門、一個包包、一段邊界，而沒有人在看著。
adjective · B2
Not watched or protected by anyone. An unguarded place, building, door or thing has nobody keeping it safe, so anyone could walk up to it or take it.
無人看守的
核心畫面
One way to hold this word: picture a gate with nobody standing at it. The gate is still there and it may even be locked, but the person whose job was to watch it has away. Nothing has gone wrong yet; what has changed is that nothing is being checked. Move that gate to your own mouth and the other uses of the word fall out: a sentence goes through that nobody looked at, and a stretch of time passes with the post empty.
可以想成一道沒有人站崗的門。門還在，甚至可能還鎖著，只是本來該守在那裡的人走開了。這時候還沒出事，改變的只是「沒有東…更多收起
可以想成一道沒有人站崗的門。門還在，甚至可能還鎖著，只是本來該守在那裡的人走開了。這時候還沒出事，改變的只是「沒有東西被檢查」。接著把那道門搬到自己的嘴巴上，另外兩個用法就出來了：一句話沒有人先看過就出去了，或者有那麼一小段時間，崗位上是空的。From the word’s history
Wiktionary gives the word as un- + guarded. The negative prefix on "guarded" is exactly the empty post in the picture above.
Wiktionary 記錄這個字是 un-（沒有）加上 guarded（被看守的）。這個否定前綴，正好就是上面那個畫面裡「空著的崗位」。
Wiktionary entry for unguarded, supplied in this packet (source_ref kaikki:3f6f69a190036ae4:1374155909:5468), etymology_text: "From un- + guarded."
Two branches
嘴巴這道門沒人站崗
把大門換成自己的嘴，畫面完全一樣：平常有人站在那裡擋住不該出去的話，這一刻沒有人在，於是話就走出去了。
adjective · C1
An unguarded remark, comment or answer is one that you make without stopping to think about how it will sound or what harm it could do.
未經思索的（話）