Postern

Part of speech: noun (n.)

1. Originally, a back door or gate; a private entrance; hence, any small door or gate. He by a privy postern took his flight. Spenser. Out at the postern, by the abbey wall. Shak.

2. A subterraneous passage communicating between the parade and the main ditch, or between the ditches and the interior of the outworks. Mahan.

Part of speech: adjective (a.)

Back; being behind; private. "The postern door." Dryden.