cumplir
/kumˈpliɾ/ · A2 · verb
to turn a certain age
🎂 cumplir = com- + plere (to fill): fill up fully, like English complete
From Latin complere, to fill up, the same root as complete, comply and accomplish. You fill a promise or duty to the brim, and cumplir años is filling up another year of age, hence the birthday use.