Parsing inflected forms may not always work as expected. If the following does not give the correct word, try Latin Words or Perseus.
prĭorsum and prĭorsus, adv. [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.