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 (abrogated in 1750), from Spanish asiento, formerly assiento "a compact or treaty; a seat in court, a seat," from asentar/assentar "to adjust, settle, establish," literally "to place on a seat," from a sentar, from Latin ad "to" (see ad-) + sedens, present participle of sedere "to sit," from PIE root *sed- (1) "to sit."