The Spanish word 'rencor' (meaning 'rancor' or 'grudge') comes from the Latin word 'rancor', which meant both 'rancidness' and 'deep-seated resentment'. This Latin noun was derived from the verb 'rancere', meaning 'to become rancid'. The metaphorical extension from physical spoilage to emotional bitterness makes intuitive sense - just as food can go bad and become rancid, feelings can also 'spoil' and turn into lasting resentment.

While 'rencor' itself is a relatively common Spanish word, there aren't many simpler or more common Spanish words that share its etymology. The word stands somewhat alone in modern Spanish vocabulary derived from this Latin root.

English speakers might recognize the connection to the English word 'rancor', which means exactly the same thing as Spanish 'rencor' - a deep-seated and bitter resentment. Both words come directly from the same Latin source 'rancor'. English also has the related adjective 'rancid', which preserves the original physical meaning of something that has gone bad or spoiled, especially referring to oils and fats that have developed an unpleasant smell or taste. This connection helps illustrate how the meaning evolved from physical spoilage to emotional bitterness.