Coming in position eighty in an ordered sequence; the ordinal number between seventy-ninth and eighty-first.
From 'eighty' (formed from 'eight' plus 'ty', representing tens) with ordinal suffix '-th'. Follows the pattern of other tens ordinals.
An 80th birthday is special because you've made it to a milestone that was considered 'very old' for most of human history—life expectancy at birth in medieval times was only about 30 years!
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