plural thirties
Definition of thirty
1
: a number equal to three times 10 — see Table of Numbers
2
thirties 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 thirty on the last page of the story
4
: the second point scored by a side in a game of tennis
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Merriam-Webster unabridged
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Time Traveler for thirty
The first known use of thirty was before the 12th century
See more words from the same century