Lewis & Short

postĕrŭla, ae, f dim. (sc. janua or via) [id.].

  1. I. A small back door or gate, a postern (post-class.): posterulae proditio, Cassian. Inst. Coenob. 5, 11.
  2. II. A back-way, by-way (post-class.), Amm. 30, 1, 13.