1. Demanding great effort or labor; difficult: "the arduous work of preparing a Dictionary of the English Language" (Thomas Macaulay).
2. Testing severely the powers of endurance; strenuous: a long, arduous, and exhausting war.
3. Hard to traverse, climb, or surmount.
[From Latin arduus, high, steep.]
The American Heritage® Dictionary of the English Language, Fifth Edition copyright ©2020 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.