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.
Origin: OF. Posterne, posterle, F. Poterne, fr. L. Posterula, fr. Posterus coming after. See Posterior.
back; being behind; private. The postern door.