lacking protection or a guard; "an unguarded gate"; "his unguarded queen was open to attack
If something is unguarded, nobody is protecting it or looking after it. I should not leave my briefcase and camera bag unguarded. = unattended
If you do or say something in an unguarded moment, you do or say it carelessly and without thinking, especially when it is something that you did not want anyone to see or know. The photographers managed to capture Jane in an unguarded moment