The Collaborative International Dictionary of English v.0.48:

Ris \Ris\, n. [AS. hr[imac]s; akin to D. rils, G. reis, OHG.
   hr[imac]s.]
   A bough or branch; a twig. [Obs.]
   [1913 Webster]

         As white as is the blossom upon the ris. --Chaucer.
   [1913 Webster]

V.E.R.A. -- Virtual Entity of Relevant Acronyms (February 2016):

RIS
       Remote Installation Service (MS, ADS)