Yours, Etc.: Origins and Uses of 8 Sign-Offs
late 14c., from Old French posterité (14c.), from Latin posteritatem (nominative posteritas) "future, future time; after-generation, offspring;" literally "the condition of coming after," from posterus "coming after, subsequent," from post "after" (see post-). Old English words for this included æftercneoreso, framcynn.