The Spanish word 'plenario' (meaning 'plenary' or 'full/complete') comes from the Late Latin word 'plenarius', meaning 'full, complete, or plenary'. 'Plenarius' itself was derived from the Classical Latin word 'plenus', meaning 'full' or 'complete'. The word maintained its meaning of fullness or completeness as it evolved from Latin through Late Latin and into Spanish.

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

English speakers might recognize the connection to words like 'plenty', 'plenary', 'plenitude', and 'replenish', which all come from the same Latin root 'plenus'. The English word 'plenty' refers to a full or abundant amount, while 'plenary' directly corresponds to the Spanish 'plenario', both referring to something that is full or complete in all aspects. 'Replenish' means to fill something up again, maintaining that core meaning of fullness from the original Latin.