The Spanish word 'nieto' (meaning 'grandson') comes from the Vulgar Latin word 'neptus', which also meant 'grandson'. This Vulgar Latin term developed from the Classical Latin word 'nepos', which had a broader meaning of both 'grandson' and 'nephew'. Over time, as the word evolved from Latin to Spanish, its meaning narrowed to specifically refer to a grandson, while losing the 'nephew' meaning component.

While 'nieto' itself is a relatively simple and common Spanish word, it's worth noting that the feminine form 'nieta' (granddaughter) follows the same etymology. Spanish students might also encounter related terms like 'bisnieto' (great-grandson) and 'tataranieto' (great-great-grandson), which are compounds formed with 'nieto'.

The English word 'nephew' is actually a cousin of the Spanish 'nieto', as both derive from the Latin 'nepos'. While 'nieto' developed to mean 'grandson' in Spanish, the English word 'nephew' retained the other meaning that was present in the original Latin word. This shows how words from the same source can evolve to take on different, though related, meanings in different languages.