Postern

[poh-stern, pos-tern] /ˈpoʊ stərn, ˈpɒs tərn/

noun
1.
a back door or gate.
2.
a private entrance or any entrance other than the main one.
adjective
3.
of, relating to, or resembling a postern.
/ˈpɒstən/
noun
1.
a back door or gate, esp one that is for private use
adjective
2.
situated at the rear or the side
n.

late 13c., “back door, private door,” from Old French posterne “side or rear gate,” earlier posterle, from Late Latin posterula “small back door or gate,” diminutive of Latin posterus “that is behind, coming after, subsequent,” from post “after” (see post-).