from The American Heritage® Dictionary of the English Language, 4th Edition
- adj. Demanding great exertion; arduous and exhausting.
from Wiktionary, Creative Commons Attribution/Share-Alike License
- adj. Of work, very physically tiring.
from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.
- adj. characterized by effort to the point of exhaustion; especially physical effort
back + breaking, that is, that breaks the back (Wiktionary)