If I recall correctly, the "dead" in "dead reckoning" comes from "de'd" which is a shortened form of "deduced". In navigation, it means deducing one's position based on the time, course, and speed from a previous known position.
No, I think it’s an old Saxon or Celtic word, can’t recall which, which just happens to be the same as the “dead” which means “lifeless”. It’s an intensifier, making the adjective to which it is attached absolute, exact.
O.E. dead, from P.Gmc. *dauthaz, from PIE *dheu-. Meaning "insensible" is first attested c.1225. Of places, meaning "inactive, dull," it is recorded from 1581. Used from 16c. in adj. sense of "utter, absolute, quite." Dead heat is from 1796. Dead reckoning may be from nautical abbreviation ded. ("deduced") in log books, but it also fits dead (adj.) in the sense of "unrelieved, absolute.
However, most uses of "dead" as an intensifier have some whacky folk theory that is wrong, so I would not be surprised if the "deduced" theory is wrong, too. Dead reckoning could simply be "exact" reckoning, not "deduced", like "dead eye" and "dead right".