Q: What is the origin of the expression “tooth and nail,” as in “They fought tooth and nail to get their way”?
A: The adverbial phrase “tooth and nail” (originally “with tooth and nail”) literally means “with the use of one’s teeth and nails as weapons; by biting and scratching,” according to the Oxford English Dictionary.
But, as the OED notes, the expression has almost always been used figuratively to mean “in the way of vigorous attack, defence, or action generally; vigorously, fiercely, with one’s utmost efforts, with all one’s might.”
In fact, the OED‘s first published reference to the phrase is figurative.
In A Dialogue of Comfort and Tribulation, which Sir Thomas More wrote while awaiting execution in 1535, he created a fictional conversation between Anthony, a wise old man, and Vincent, a young man fearful that invading Turks might kill him if he didn’t betray his Christian faith.
Anthony argues that some men love the delights of the world so much that “they would faine kepe them as long as ever they might, even with tooth and naile,” but thereby lose the greater gifts of heaven.
We seem to have a thing about teeth and nails, which show up in other figurative expressions.
A few toothy ones are “show one’s teeth” (show hostility), “get one’s teeth into something” (begin serious work on it), and “lie through one’s teeth” (tell a deliberate whopper).
Some phrases featuring nails are “bite one’s nails” (be nervous), “drive the nail home” (clinch an argument), and “hit the nail on the head” (get to the heart of the matter).
I hoped I’ve nailed this answer to your satisfaction.