The Spanish word 'plenaria' comes from the Late Latin word 'plenarius' meaning 'full or complete', which itself derives from the Latin word 'plenus' meaning 'full'. This evolution shows how the basic concept of fullness or completeness has been preserved through the word's history from Latin to modern Spanish.

Some related Spanish words include 'pleno' (full, complete), 'plenitud' (fullness, completeness), and 'llenar' (to fill). All these words share the same Latin root 'plenus' and maintain related meanings around the concept of fullness or completeness.

English speakers might recognize related words like 'plenty', 'plenary', and 'plenitude', which all come from the same Latin root 'plenus'. The English word 'plenary' is particularly close to the Spanish 'plenaria', as both refer to something that is full or complete in all aspects. The word 'plenty' developed from the same root to mean 'a full amount' or 'abundance'.