Unguarded means left unprotected, or said without thinking. An unguarded moment, an unguarded remark, left unguarded.
The second sense is the commoner: an unguarded remark is one that slipped out, and in an unguarded moment is the standard excuse for it. Of a place it means literally without a guard. Note guard; and that the metaphor treats speech as something that has to be watched.
1adjectivethinkingSaid without thinking.
“The minister's unguarded comment to a reporter caused a scandal the next day.”
2adjectiveunguardedLeft unprotected.
“The thieves got in through an unguarded door at the back of the museum.”
unguarded is a quality of being unprotected.
From un- + guarded.
Etymology from Wiktionary, CC BY-SA