Meaning of underwire

un•der•wire

Pronunciation: (un'dur-wīr"), [key]
— n.
  1. a wire sewn into the underside of each cup of a brassiere, used for support and shape.
  2. a brassiere with such wires.
Random House Unabridged Dictionary, Copyright © 1997, by Random House, Inc., on Infoplease.