It actually was originally called "duck tape" (like the brand name) because a) it was made from cotton duck (from "doek" for you Dutch folks) and b) they made it for soldiers to use during World War II. The soldiers could wrap their guns and ammo cases in it to keep the water out-- so the water would slip off "like water off a duck's back."
After the war, a lot of G.I.s got into HVAC work and, being familiar with it, knew duck tape had a myriad of uses, so they began using it for duct work, construction and the like. Thus, it became more popularly known as "duct tape."
Source: Works for the company who owns the original Duck Tape brand and has had a conversation with the head of marketing in his office.
I'll take that Most Pedantic award now.