Emily Dickinson Lexicon

POST-ERN, n. [Fr. pôterne, for posterne, from L. post, behind.]

1. Primarily, a back door or gate; a private entrance; hence, any small door or gate. – Dryden. Locke.
2. In fortification, a small gate, usually in the angle of the flank of a bastion, or in that of the curtain or near the orillon, descending into the ditch. – Encyc.