The Spanish word 'feliz' meaning 'happy' comes from the Latin word 'felix', which meant both 'fertile' and 'happy'. This Latin term evolved from the Proto-Italic root '*felwi-', meaning 'fertile' or 'productive'. The semantic shift from 'fertile/productive' to 'happy' makes intuitive sense, as prosperity and abundance often lead to happiness and well-being. Over time, as the word evolved from Latin to Spanish, it retained the 'happy' meaning while dropping the agricultural connotation of fertility.

The Spanish word 'felicidad' (happiness) and 'felicitaciones' (congratulations) are common related words that share the same root. These words are often encountered earlier in Spanish learning than 'feliz' and can help remember the connection to happiness and well-being.

In English, we see this same Latin root in words like 'felicity' (great happiness), 'felicitous' (well-chosen or suited to the circumstances), and 'felicitate' (to congratulate). The word 'felix' is also used as a given name, originally bestowed with the wish for the child to be happy and prosperous.