Lewis Short
postĕrŭla, ae
* F dim. (sc. janua or via) [id.].
* A small back door or gate, a postern (post-class.): posterulae proditio, Cassian. Inst. Coenob. 5, 11.
* A back-way, by-way (post-class.), Amm. 30, 1, 13.
Charlton T. Lewis, Charles Short, A Latin Dictionary