from The American Heritage® Dictionary of the English Language, 4th Edition
- adj. Demanding great effort or labor; difficult: "the arduous work of preparing a Dictionary of the English Language” ( Thomas Macaulay).
- adj. Testing severely the powers of endurance; strenuous: a long, arduous, and exhausting war.
- adj. Hard to traverse, climb, or surmount. See Synonyms at burdensome, hard.
from Wiktionary, Creative Commons Attribution/Share-Alike License
- adj. Needing or using up much energy; testing powers of endurance.
- adj. burning; ardent
- adj. this sense) Difficult or exhausting to traverse.
from the GNU version of the Collaborative International Dictionary of English
- adj. Steep and lofty, in a literal sense; hard to climb.
- adj. Attended with great labor, like the ascending of acclivities; difficult; laborious.
from The Century Dictionary and Cyclopedia
- Steep, and therefore difficult of ascent; hard to climb.
- Attended with great labor, like the ascent of acclivities; difficult.
- Hence—3. Energetic; laborious: said of persons or actions: as, an arduous student;
from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.
- adj. characterized by effort to the point of exhaustion; especially physical effort
- adj. difficult to accomplish; demanding considerable mental effort and skill
- adj. taxing to the utmost; testing powers of endurance
From Latin arduus, high, steep.(American Heritage® Dictionary of the English Language, Fourth Edition)
From Latin arduus ("lofty, high, steep, hard to reach, difficult, laborious"), akin to Irish ard ("high"). (Wiktionary)