The Spanish word 'cumpleaños' (birthday) is a compound word formed from two Latin roots. The first part comes from the Latin verb 'complere' meaning 'to fill completely', which evolved into the Spanish verb 'cumplir' (to fulfill). The second part comes from the Latin word 'annus' meaning 'year', which became 'año' in Spanish. When combined, 'cumpleaños' literally means 'fulfillment of years' or 'completion of years' - the day when one completes another year of life.

Some simpler Spanish words related to 'cumpleaños' include 'cumplir' (to fulfill, to accomplish, to complete) and 'año' (year). The verb 'cumplir' is used in many common expressions like 'cumplir una promesa' (to keep a promise) or 'cumplir con su deber' (to fulfill one's duty). 'Año' is one of the most basic Spanish words for talking about time.

While English 'birthday' takes a different etymological path, there are some English words related to the Latin roots of 'cumpleaños'. From Latin 'complere', we get English words like 'complete', 'complement', and 'accomplish'. From Latin 'annus', we get words like 'annual', 'anniversary', and 'perennial'. Understanding these connections can help remember that 'cumpleaños' is about the completion or fulfillment of another year.