Overly well equipped or prepared, as in With her elaborate gown and makeup, she was armed to the teeth for her first New York appearance. The expression to the teeth meant “well equipped” in the 14th century, when knights often wore head-to-foot armor. The idiom, however, only gained currency in the mid-1800s, at first still applied to weapons or other military equipment. Today it is used still more figuratively.
The American Heritage® Idioms Dictionary Copyright © 2002, 2001, 1995 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company.