Likely a semi-learned word, which became well-ingrained in the popular language at an early date, taken from Latin voluntātem (cf. also Old Spanish form veluntad), accusative singular form of voluntās (“will”, “desire”, “disposition toward”); however, as no descendant of the related voleō exists in Spanish either, this makes sense. Portuguese vontade and Old Catalan volentat may be inherited, however. Compare also the English volunty, the French volonté, and other cognates at voluntas#Descendants.
voluntad f (plural voluntades)