The Spanish word 'hijo' (meaning 'son') comes from the Old Spanish word 'fijo', which in turn derives from the Latin word 'filius' (also meaning 'son'). The evolution from Latin to modern Spanish shows a common sound change pattern where the Latin 'f' transformed into an 'h' sound in Spanish (a process called 'f- to h- lenition'), and the Latin '-lius' evolved into '-jo', following regular Spanish phonological development.

Some related Spanish words include 'filial' (meaning 'relating to a son or daughter'), which preserved the original Latin 'f-' because it was borrowed later as a cultured word. The feminine form 'hija' ('daughter') follows the same etymological pattern, and you'll also find 'hijastro/hijastra' ('stepson/stepdaughter') which combines 'hijo/hija' with the suffix '-astro/a' indicating a family relationship by marriage.

English speakers might recognize the connection to words like 'filial' (relating to or due from a son or daughter) and 'affiliate' (to connect or associate with), which come from the same Latin root 'filius'. The word 'filho' in Portuguese is also a cognate, showing how different Romance languages evolved from the same Latin source.