Source : Webster's Revised Unabridged Dictionary (1913)

Arsis \Ar"sis\ ([aum]r"s[i^]s), n. [L. arsis, Gr. 'a`rsis a
   raising or lifting, an elevation of the voice, fr. a'i`rein
   to raise or lift up. Its ordinary use is the result of am
   early misapprehension; originally and properly it denotes the
   lifting of the hand in beating time, and hence the unaccented
   part of the rhythm.]
   1. (Pros.)
      (a) That part of a foot where the ictus is put, or which
          is distinguished from the rest (known as the thesis)
          of the foot by a greater stress of voice. --Hermann.
      (b) That elevation of voice now called {metrical
          accentuation}, or the rhythmic accent.

   Note: It is uncertain whether the arsis originally consisted
         in a higher musical tone, greater volume, or longer
         duration of sound, or in all combined.

   2. (Mus.) The elevation of the hand, or that part of the bar
      at which it is raised, in beating time; the weak or
      unaccented part of the bar; -- opposed to {thesis}.
      --Moore.