The Spanish word 'lleno' meaning 'full' comes from the Latin word 'plenus' which meant 'full' or 'complete'. This is a clear example of the common Latin 'pl-' to Spanish 'll-' sound change that occurred as Latin evolved into Spanish. The meaning has remained remarkably stable over time, continuing to express the concept of fullness.

Some related Spanish words include 'llenar' (to fill), 'relleno' (stuffed/filled), and 'plenitud' (fullness). While 'llenar' and 'relleno' followed the same sound change as 'lleno', 'plenitud' is a more learned borrowing that retained the original Latin 'pl-' sound.

Several English words are related to 'lleno' through the shared Latin ancestor 'plenus'. These include 'plenty' (abundance), 'plenary' (full, complete), 'replenish' (to fill again), and 'plentiful'. All these English words retained the original Latin 'pl-' sound and share the core meaning of fullness or completeness.