Source : Webster's Revised Unabridged Dictionary (1913)

Siamang \Si"a*mang`\, n. [Malay si[=a]mang.] (Zool.)
   A gibbon ({Hylobates syndactylus}), native of Sumatra. It has
   the second and third toes partially united by a web.

Source : WordNet®

siamang
     n : large black gibbon of Sumatra having the 2nd and 3rd toes
         partially united by a web [syn: {Hylobates syndactylus},
         {Symphalangus syndactylus}]