• Etymology Today from M-W: diluvial \duh-LOO-vee-ul
: of, relating to, or brought about by a flood

Late Latin "diluvialis" means "flood." It's from "diluere" ("to wash away") and ultimately from "lavere" ("to wash"). English "diluvial" and its variant "diluvian" initially referred to the Biblical Flood. Geologists, archaeologists, fossilists, and the like used the words, beginning back in the mid-1600s, to mark a distinct geological turning point associated with the Flood. They also used "antediluvian" and "postdiluvian" to describe the periods before and after the Flood. It wasn't until the 1800s that people started using "diluvial" for floods and flooding in general. American educator and essayist Caroline M. Kirkland, one early user of this sense, wrote, "Much of our soil is said to be diluvial — the wash of the great ocean lakes as they overflowed towards the south," in her essay Forest Life in 1850.

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