The quality of being fast or quick; swiftness and speed. Also, a secure or fortified place; a stronghold.
From 'fast' (adjective meaning quick or firm) plus '-ness' (the noun-forming suffix creating abstract qualities). The second meaning derives from 'fast' meaning 'secure' or 'firmly fixed.'
This word brilliantly has two meanings: the fastness of a cheetah (its speed) and the fastness of a medieval fortress (its security)—both come from the same root idea of something being firmly established!
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