Back Door, Rear Entrance
noun, の adjective
The backside mouth (think: entrance) is the back door or rear entrance.
うらぐち
The reading is 裏 and 口 put together into one word.
彼女の家には隠された裏口があるという彼の話は、信憑性を増した。
His story that there is a secret backdoor to her house grew in credibility.