assiento

n.

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).

Online Etymology Dictionary, © 2010 Douglas Harper

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Historical Examples of assiento