I have to admit that when I first encountered the term, I thought that “dead reckoning” must be derived from “deduced reckoning”. I’ll bet that the WWII flyboys felt the same way – regardless of the “real” origin (which seems vague enough in your piece), the people who use the term know where they think it comes from, and how they interpret it. This has, I think, a certain validity to it – the term might have gobe the way of a great many other terms and become obsolete, replaced by some other, newer term, if it were not for the fact that all of these modfern navigators had continued to use the term, reassured by the belief that it stood for “deduced” reckoning.
I’m not trying to use sophistry here – language is a living thing, influenced by many factors, and I think it would be wrong for someone to come along and say that “dead” reckoning only comes from “dead in the water” reckoning or something like that – the term has been kept alive and in use by people who really think it comes from “deduced” reckoning.
To give another example, in Tom Burnham’s wonderful Dictionary of Misinformation he talks of how “belfry” (as in “bats in your ____”) derives from an original form “berfry”. So, say some people, it has nothing to do with the “bell” that’s kept in the belfry at all. But, of course, it does. Generations of people have continued to use the “incorrect” term “Belfry” rather than “berfry” (or instead of inventing another term, like “bell tower”) precisely because they thought it had something to do with “bell”.