The Spanish word 'relleno' (meaning 'filled' or 'stuffed') comes from the verb 'rellenar' meaning 'to fill again' or 'to stuff'. The verb 'rellenar' was formed by combining two elements: the prefix 're-' meaning 'again' and the word 'lleno' meaning 'full'. The word 'lleno' itself comes from Latin 'plenus' meaning 'full'. The evolution from Latin 'plenus' to Spanish 'lleno' shows a common sound change in Spanish where the initial 'pl-' cluster became 'll-'.

A simpler and very common Spanish word related to 'relleno' is 'lleno' meaning 'full'. While 'lleno' directly describes something being full, 'relleno' specifically refers to something that has been filled or stuffed, like in 'chile relleno' (stuffed chile pepper). The prefix 're-' adds the notion of filling something up again or filling it completely.

The English words 'plenty' and 'plenary' are related to 'relleno' as they also come from Latin 'plenus'. While 'plenty' evolved to mean 'a large amount' or 'abundance', and 'plenary' means 'full' or 'complete' (as in 'plenary session'), the Spanish 'relleno' specialized to mean specifically 'filled' or 'stuffed'. These words showcase how the same root evolved differently in English and Spanish while maintaining connections to the original concept of fullness.