n. pl. for·ties
1. The cardinal number equal to 4 × 10.
a. A decade or the numbers from 40 to 49: They stopped smoking in their forties. At night the temperature fell into the forties.
b. often Forties The decade from 40 to 49 in a century.
[Middle English, from Old English fēowertig; see kwetwer- in the Appendix of Indo-European roots.]
forty adj. & pron.
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