There is a small but noticeable subset of Latin positive adverbs that end in -us. (By "positive," I am excluding the standard comparative adverb form of -ius, e.g. citius.)
Some examples that come immediately to mind are penitus, protinus, caelitus, divinitus, and (my favorite) medullitus. I also appended a list of such adverbs at the bottom of this question, though some (like adversus) are simply taken from the corresponding adjective.
Can anyone elucidate why these adverbs formed in this way?
Complete list:
adversus, advorsus, aliorsus, aliovorsus, aliquatenus, alterinsecus, alterius, alterorsus, altrimsecus, altrinsecus, antiquitus, caelitus, circumsecus, claritus, coelitus, cominus, comminus, communitus, conmunitus, controversus, cordetenus, cordicitus, corsus, deintus, demus, demus, deorsus, deversus, dextrorsus, dextroversus, dextrovorsus, dius, divinitus, eatenus, eminus, exadversus, exterius, extrinsecus, forinsecus, funditus, fundius, hactenus, humanitus, interdius, intrinsecus, introrsus, introsus, intus, laevorsus, medullitus, mordicus, nihilominus, nominetenus, nudius, nudiustertius, nullatenus, penitus, peropus, perpensius, potius, primitus, prius, prorsus, prosus, protinus, publicitus, quadamtenus, quadantenus, quamtocius, quamtotius, quandius, quantocius, quatenus, quatinus, quoquoversus, quorsus, radicitus, retrorsus, retroversus, rursus, secius, secus, seorsus, sequius, seriosius, serius, setius, sinisbrorsus, sinisbrosus, sinistrorsus, solemnitus, sollemnitus, subtus, superficietenus, terratenus, ullatenus, undiquesecus, undiquesecus, utrimquesecus, versus, vorsus,