1714, "contract between the King of Spain and another power" (especially that made at the Peace of Utrecht, 1713, with Great Britain for furnishing African slaves to the Spanish colonies in the Americas), from Spanish asiento, from asentar "to adjust, settle, establish," literally "to place on a chair," from a sentar, from Latin sedens, present participle of sedere "to sit" (see sedentary).
Examples from the Web for assiento
To estimate the extent of the smuggling trade directly traceable to the loop-holes which the Assiento offered, was impossible.The History of Cuba, vol. 2|Willis Fletcher Johnson
This contract was technically called in those days an assiento.
A good ministry would have considered how a renewal of the Assiento might have been obtained.
The importation then dwindled, but rose after the Assiento to perhaps 30,000.