Dreadnought is a boast squeezed into one word. It joins dread, "to fear," with nought, "nothing," to make "fear nothing." Etymonline traces the idea to a Middle English phrase, drede ich nawiht, "I dread nothing," recorded around 1200.
English sailors liked the sound of it. Wikipedia's list of Royal Navy ships called Dreadnought begins with one built in 1553 and another launched in 1573. The 1573 ship was rebuilt twice and lasted until 1648. Wordsmith.org gives 1573 as the word's earliest documented use, while Etymonline first records the ship name around 1596. The name was then reused for centuries, on ships of 1691, 1742 and 1801, among others.
The ship that changed the word's meaning was the one launched in February 1906. Its keel was laid in October 1905, it ran sea trials in October 1906 and it was completed in December 1906, very fast for a battleship. Admiral Sir John "Jackie" Fisher, First Sea Lord from 1904, is credited with the design.
Its novelty was simple to state. Most battleships of the day carried four big guns. Dreadnought carried ten 12-inch guns. It used steam turbines instead of piston engines and reached 21 knots, about three knots faster than its rivals.
The effect was immediate. Royal Museums Greenwich notes that its armament made every other battleship of the era obsolete. Later ships of the same design were called dreadnoughts, and the older ones became known, after the fact, as pre-dreadnoughts. Within five years even bigger designs, such as Britain's Orion class of 1910, earned the name super-dreadnoughts. Germany built its own fleet to match, and the rivalry produced the two largest dreadnought fleets before 1914.
Etymonline dates the generic sense, "a battleship of this type," to 1916. The term faded after the First World War, once nearly every battleship followed the same pattern.
The word kept spreading. Wordsmith.org lists a thick cloth and a warm coat made from it, worn at sea against the weather. It also lists a large acoustic guitar with a big body and loud sound. C.F. Martin introduced that style in 1916 and named it after the 1906 battleship.
The navy has not let the name go. In 1960 it went to Britain's first nuclear-powered submarine, and a new class of ballistic missile submarines is the Dreadnought class.