The Collaborative International Dictionary of English v.0.48:

Tucum \Tu"cum\, n. [So called by the Indians of Brazil.]
   A fine, strong fiber obtained from the young leaves of a
   Brazilian palm (Astrocaryum vulgare), used for cordage,
   bowstrings, etc.; also, the plant yielding this fiber. Called
   also tecum, and tecum fiber.
   [1913 Webster]