Llenar comes from the Latin plere (“to fill”), as we’ve previously discussed. But here’s another English word that comes from the same Latin root: expletive, yes, that euphemism for vulgar words!
Expletive literally means to “fill” with the expansive ex– prefix which, taken together, mean, “to fill out your words.” An expletive is literally filling conversation with words when you don’t know what else to say!
The Spanish lluvia (for “rain”) comes Latin pluvia for the same — a change that may not be obvious because the -pl- of Latin sometimes became a -ll- in Spanish.
From the same root, we get the sophisticated English word pluvial which means… lots of rain!
The ll-v of lluvia clearly maps to the p-l of pluvial.
Llenar — Spanish meaning “to fill” — comes from the Latin plenus, meaning “full”.
This, therefore, connects it to the English for the same, from the same root: Plenty. Not to mention, the less common English word plenary.
These words sound so different yet they’re so similar. Here’s how: Latin words that began with pl- usually turned into ll- when Latin evolved into Spanish. But as these words moved into English via French, they remained unchanged.
This explains not just llenar/plenty but explains a bunch of other words, including llama/flame.