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.
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!
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.