The story
Uncertain. Not short for deduced or deductive, but this belief explains the alternative spelling ded reckoning (which actually originated centuries later).
Etymology from Wiktionary, CC BY-SA 4.0.
dead reckoning
Today it means: A method of estimating the position of a ship or aircraft by applying estimates of the distance and direction travelled from a previously known position. In respect to ships/boats, it excludes the effect of wind and current on the vessel. Full definition →