Source : Webster's Revised Unabridged Dictionary (1913)

Assiento \As`si*en"to\, n. [Sp. asiento seat, contract or
   agreement, fr. asentar to place on a chair, to adjust, to
   make an agreement; a (L. ad) + sentar, a participial verb; as
   if there were a L. sedentare to cause to sit, fr. sedens,
   sedentis, p. pr. of sed?re to sit.]
   A contract or convention between Spain and other powers for
   furnishing negro slaves for the Spanish dominions in America,
   esp. the contract made with Great Britain in 1713.