Postern /(?)/

Pos·tern

Postern

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.
    Out at the postern, by the abbey wall.
  2. A subterraneous passage communicating between the parade and the main ditch, or between the ditches and the interior of the outworks. (Fort.)

Postern

a.
  1. Back; being behind; private.