Meaning

noun a rate (usually rapid) at which something happens

noun the quality of being fixed in place as by some firm attachment

noun a strongly fortified defensive structure

originFrom Middle English fastnes, fastnesse (“immovableness, stability; firmness, solidness; secure attachment; constancy; obstinacy; something giving firmness or strength; of sleep: soundness; of a wound: healing; assurance, confirmation”) [and other forms], from Old English fæstnes (“fastness, firmness, stability; resolution; tenacity; vigour; firmament; fortification”) [and other forms], from fæst…

Etymology from Wiktionary, CC BY-SA 3.0