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.
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."
登入就可以看每一支怎麼連回核心畫面。