The Collaborative International Dictionary of English v.0.48:

must \must\, musth \musth\(m[u^]st), a. [Hind. mast intoxicated,
   ruttish, fr. Skr. matta, p.p. of mad to rejoice, intoxicate.]
   (Zool.)
   Being in a condition of dangerous frenzy, usually connected
   with sexual excitement; -- said of adult male elephants which
   become so at irregular intervals, typicaly due to increased
   testosterone levels. -- n.
   (a) The condition of frenzy.
   (b) An elephant in must.
       [Webster 1913 Suppl. +PJC]

WordNet (r) 3.0 (2006):

musth
    n 1: an annual phase of heightened sexual excitement in the
         males of certain large mammals (especially elephants); is
         associated with discharge from a gland between the eye and
         ear; "the frenzied elephant was in musth"