where the word came from
who invented it

Britain, the name means "Fearless."

The English used the word for ship names going back at least 500 years, but it's most famous application was for the world's first all big-gun battleship, HMS Dreadnaught, built in 1905.

I think that one of the ships in Howard's fleet that fought the Spanish Armada was named "Dreadnaught."

Fear God and dread naught.

HMS Dreadnaght was indeed one of the royal warships during the Armada campaign - along with many others. Off the top of my mind, the "HMS" included Ark Royal (Howard), Revenge (Drake), Victory (Hawkyns), Triumph (Frobisher), Rainbow (either Wynter or Seymour, not sure), Vanguard (the other of these two), Nonpareil, Swallow, Mary Rose, etc.