adverb : fully, completely
Scarcely had his hideous laugh rang out but once, when I was upon him. The brute was twelve feet in height and armed to the teeth, but I believe that I could have accounted for the whole roomful in the terrific intensity of my rage.
— Edgar Rice Burroughs, A Princess of Mars, 1917
This phrase most often describes how someone is armed: we can only assume that if the teeth are involved in preparation for battle, surely it's only after all other weapons are ready. A similar but less common phrase, to one's teeth, means "to one's face; openly." In Shakespeare's "Hamlet" Laertes tells the King "It warms the very sickness in my heart / That I shall live and tell him to his teeth, / 'Thus didest thou.'"