from The American Heritage® Dictionary of the English Language, 4th Edition
- adv. With every available resource; with unrelenting effort: "Bureaucrats would correctly see this as a curb on their power and would fight tooth and nail against it” ( George R. Packard).
from Wiktionary, Creative Commons Attribution/Share-Alike License
- adv. Viciously; with all one’s strength or power; without holding back.
from the GNU version of the Collaborative International Dictionary of English
- n. as if by biting and scratching; with one's utmost power; by all possible means.
from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.
- adv. with force and ferocity
From tooth + and + nail ("fingernail, claw"). (Wiktionary)