" The paces that English puts damn through are rather astounding. Darn, for example, is not a random fudging but a downright game-of-telephone mangling of what began as By the eternal! as a euphemism for by the eternal God. There were those who were given to pronouncing the word etarnal, for the same reason that they might say “larn” for learn—or, for that matter, pronounce concern as “consarn” in Consarn it!, yet another euphemism for goddamnit. That etarnal shortened, naturally, to tarnal. Because this was a substitute for damn, it was equally natural to assume subconsciously that if there is a word damnation there is a word tarnation—and soon, there was. From here, it was a short step to imagining that if damnation had its damn, then tarnation had its tarn, or, since what we really have in mind is a way of saying damn without saying it and damn begins with d, darn. Few etymologies rival this one in the contrast between the beginning and the end, such as the origins of bye in God be with you. "

― John McWhorter , Nine Nasty Words: English in the Gutter — Then, Now, and Forever