Lewis Short
(adverb) : prÄorsum and prÄorsus, prisversus
* Forwards (post-class.): corpus aut accedit priorsum, aut retrorsum recedit (al. prorsum),Macr. S. 7, 9: pergere,id. ib. 7, 13: movetur corpus priorsus ac retrorsus,Claud. Mam. Stat. Anim. 1, 18.
Charlton T. Lewis, Charles Short, A Latin Dictionary