'assiento' originates from Spanish, specifically the word 'asiento', where the verb 'asentar' (from Latin roots) meant 'to settle' or literally 'to make sit'.

'assiento' changed from Spanish 'asiento' (itself from medieval Spanish forms related to 'asentar') and was borrowed into English in the 17th–18th centuries as 'assiento' or 'asiento', coming to be used particularly for the slave‑supply contract.

Initially it meant 'seat' or 'a settled place/settlement' and by extension 'a settled arrangement or grant'; in English the term narrowed to refer chiefly to the specific contractual grant (the asiento) for supplying enslaved people.