This first referred to a 41-gun galleon in the English Navy Royal, launched in 1573 (according to Wikipedia), and has since been used to refer to a number of things, including 20th Century battleships and acoustic guitars.
Literally ‘fear nothing’.
But, somehow, this word creates its own frisson of fear.
It’s probably a combination of the fact that something that ‘fears nothing’ is going to be a pretty big, scary thing itself, and the combination of the actual words dread + nought.
I mean, ‘dread’ isn’t just fear, it’s fear levelled up some. And ‘nought’ is a nice, tasty, archaic way of saying ‘nothing’, which gives the term a certain historical weight.