There seems to be some disagreement as to whether it started as "Duck" tape and changed to "Duct" tape after WW-II, or the other way around.

Oddly enough, "Duct" tape is not a very good tape to use on ducts...

"The origin of the name of the product, "duck tape" or "duct tape", is the subject of some disagreement. One view [14] is that it was called "duck tape" by WWII soldiers either because it resembled strips of cotton duck or because the waterproof quality of the tape contributed to the name, by analogy to the water-shedding quality of a duck's plumage. Under this view, soldiers returning home from the war found uses for duck tape around the house, where tents were forgotten and ductwork needed sealing, not ammunition cases. Other proponents of this view point to older references to non-adhesive cotton duck tape used in Venetian blinds, suggesting that the name was carried over to the adhesive product. The Oxford English Dictionary says that perhaps "duct tape" was originally "duck tape". This view is summarized most notably in a New York Times article by etymologist William Safire in March 2003. Safire cites use of the term "cotton duck tape" in a 1945 advertisement for surplus government property.[15] The Oxford English Dictionary gives a 1902 quotation for "100,000 yards of cotton duck tape" being used to protect the cables of the Brooklyn Bridge.[16] Thus a fabric duck tape was available to which an adhesive could have been added.
In any case, whether it is an error or a preservation of the original usage, the term "duck tape" is used for the product today.[17] Duck Tape is also a brand name for this product in some countries."


Duct tape - Wikipedia, the free encyclopedia


P.S. I know the failings of Wikipedia, but I have seen this many other places before as well, and Wiki seemed to have the best summary.