The Collaborative International Dictionary of English v.0.48:

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.
          [1913 Webster]

   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.
         [1913 Webster]

   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.
      [1913 Webster]