Merriam-Webster Collegiate Dictionary - thirty

 noun  (plural thirties)  Etymology: Middle English thritty, from thritty, adjective, from Old English thritig, from thritig group of 30, from thrie three + -tig group of ten; akin to Old English tien ten  Date: before 12th century  1. — see number table  2. plural the numbers 30 to 39; specifically the years 30 to 39 in a lifetime or century  3. a sign of completion ; end — usually written 30 wrote ~ on the last page of the story  4. the second point scored by a side in a game of tennis  • thirtieth adjective or noun  • ~ adjective  • ~ pronoun, plural in construction  • ~ish adjective