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.
The American Heritage® Dictionary of the English Language, Fifth Edition copyright ©2018 by Houghton Mifflin Harcourt Publishing Company. All rights reserved.
Indo-European & Semitic Roots Appendices
Thousands of entries in the dictionary include etymologies that trace their origins back to reconstructed proto-languages. You can obtain more information about these forms in our online appendices:
The Indo-European appendix covers nearly half of the Indo-European roots that have left their mark on English words. A more complete treatment of Indo-European roots and the English words derived from them is available in our Dictionary of Indo-European Roots.