Fearnought, most properly, was a thick woollen cloth often used on sailing ships in the late 1700s through the 1800s. It was used for all sorts of things -- covering portholes, helping to line and seal the door to the powder magazine, and most particularly, protecting the sailors from the weather.
Which brings us to the most common meaning of the word fearnought, AKA a dreadnaught; this thick cloth was used to make the thick cloaks or jackets (and less frequently, trousers), usually dyed blue, that were traditionally worn by British sailors.
Fearnought was a rough fabric, with a long pile and sometimes felted. It was most certainly not a fashionable fabric, and the coats made from it were likewise not fashionable. However, it was quite practical, and did the job of keeping sailors comparatively warm and dry.
Fearnought cloth was preceded by oilskin (linen or cotton soaked in oil), and was supplanted by waxed cotton, among other more modern fabrics.