Language Of Confusion: Red-

Words descended from the Proto Indo European root red-, which means to scrape, scratch, or gnaw, and pops up in a surprising number of places.
 
First, rodent. It showed up fairly recently, in 1835, from the Latin rodentia, which was used as the name of the Order of gnawing mammals. It’s from the classical Latin rodentem, gnawing, which is from red-. The word rat itself might be related—I mean, it makes sense, but you know how these go. It’s from the Old English raet, and no one knows exactly where that came from. It might be from the Vulgar Latin rattus (come on, you’d think that would have to be it), or it might be taken right from rodere. Or, you know, not.
 
Erode showed up in the early seventeenth century while erosion showed up earlier, in the mid sixteenth century. Both come from the classical Latin erodere, to erode, which is a mix of the prefix ex-, away, and rodere, to gnaw. To erode is to gnaw away. Rodent and erode, two words I wouldn’t think would be related but am somehow not surprised that they are.
 
Then there’s corrode, which showed up in the late fourteenth century, making it older than the erode words, and for that matter corrosion. Corrode is from the Old French corroder and classical Latin corrodere, to corrode, while corrosion is from the noun form, corrosionem. This time the prefix is com-, though it’s just thought to be intensive here, and with rodere, to corrode is to really gnaw at something.
 
Finally today, we’ll look at rash—as in a skin break out, not impulsivity, which is from a different origin. Well, most likely anyway, as the origin of skin rash isn’t certain. It showed up in the early eighteenth century and might be from the French rache, which means a scratch or sore. That’s from the Vulgar Latin rasicare, to scrape, from the classical Latin rasitare, to shave, and that’s from radere and red- of course. Well, you do scratch at rashes…
 
Sources
Online Etymology Dictionary
Google Translate
Omniglot
University of Texas at Austin Linguistic Research Center
University of Texas at San Antonio’s page on Proto Indo European language
Tony Jebson’s page on the Origins of Old English
Old English-English Dictionary
Encyclopaedia Britannica