Meaning of the word "a postern door makes a thief" in English

What does "a postern door makes a thief" mean in English? Explore the meaning, pronunciation, and specific usage of this word with Lingoland.

a postern door makes a thief

US /ə ˈpoʊ.stərn dɔːr meɪks ə θiːf/
UK /ə ˈpɒ.stən dɔː meɪks ə θiːf/
"a postern door makes a thief" picture

Idiom

an unlocked or poorly secured back door provides an easy opportunity for someone to steal; opportunity creates a thief

Example:
You should lock the back gate, because a postern door makes a thief.
Leaving your laptop in an unlocked car is risky; remember that a postern door makes a thief.