A backdoor or rear entrance; the exit/entrance at the back of a building rather than the front.
配達員は裏口から荷物を運び入れた。
The delivery person carried the package in through the back door.
この店は正面より裏口の方が狭い。
This shop's back entrance is narrower than the front.
イベント会場のスタッフは裏口で出入りする。
Staff at the event venue come and go through the back entrance.
An illicit or unauthorized method; doing something unofficially or illegally (e.g., '裏口入学'—admission by illicit means).
有名大学への裏口入学が問題になった。
Admission to the prestigious university by illicit means became a problem.
その企業は裏口での契約が多いと噂されている。
There are rumors that the company often makes contracts through backdoor deals.
裏口で入社した社員は周りの信頼を失うことがある。
Employees hired through backdoor methods may lose the trust of their colleagues.