There are 34 meanings listed in oed's entry for the noun brace, 11 of which are labelled obsolete. See ‘meaning & use’ for definitions, usage, and quotation evidence. Definition of brace noun in oxford advanced learner's dictionary.

A brace is a metal device that can be fastened to a child's teeth in order to help them grow straight. Something that holds parts together or in place, as a clasp or clamp. See examples of brace used in a sentence. A brace is a support that helps to steady or strengthen, and to brace is to prepare for something hard. So brace yourself for that impromptu meeting with your boss if you haven't been performing well. To prepare or position so as to be ready for impact or danger: Union members braced themselves for a confrontation with management.

So brace yourself for that impromptu meeting with your boss if you haven't been performing well. To prepare or position so as to be ready for impact or danger: Union members braced themselves for a confrontation with management. To get ready for something difficult or unpleasant (chiefly us) usually + for usually + for; To give added physical support or strength to (something)