'asiento' originates from Latin, specifically from a Vulgar/late-Latin formation related to 'adsentāre' (or 'adsidere'), where the element 'ad-' meant 'to/toward' and the root 'sedēre' meant 'to sit'.

'asiento' changed from a Vulgar Latin form like '*adsentāre' into Old Spanish 'assiento' and eventually became the modern Spanish word 'asiento'.

Initially it meant 'an act of sitting' or 'a place to sit'; over time it broadened to include 'assigned place/ticket', 'accounting entry', and (historically) 'a formal contract or grant'.