The Spanish word 'nochevieja' (meaning 'New Year's Eve') is a compound word formed from two parts: 'noche' ('night') and 'vieja' ('old'). The first part comes from Latin 'noctem' meaning 'night', which evolved directly into Spanish 'noche'. The second part has a more complex evolution: it starts with Latin 'vetus' meaning 'old', which became the diminutive form 'vetulus', then transformed into Vulgar Latin 'veclus', and finally became Spanish 'viejo/vieja'. When combined, 'nochevieja' literally means 'old night', referring to the last night of the old year.

Several common Spanish words are related to 'nochevieja'. The word 'noche' by itself is a very basic Spanish word meaning 'night' that students learn early on. Similarly, 'viejo/vieja' meaning 'old' is another fundamental Spanish adjective. Understanding these simpler components makes it easier to grasp why Spanish speakers call New Year's Eve 'nochevieja' - it's literally the 'old night' or the final night of the year.

While English 'night' and Spanish 'noche' might not look very similar at first glance, they are actually distant cousins, both descending from the Proto-Indo-European root *nekʷt-. The English word 'nocturnal' is more obviously related, as it comes from the same Latin root 'noctem' that gave Spanish 'noche'. Similarly, the English word 'veteran' is related to the 'vieja' part of 'nochevieja', as it comes from Latin 'vetus' meaning 'old'.