counterbrace

from The Century Dictionary.

• noun Nautical, the lee brace of the foretopsail-yard.
• noun In a frame, a brace which transmits a strain in an opposite direction from a main brace.
• Nautical, to brace in opposite directions: as, to counterbrace the yards (that is, to brace the head-yards one way and the after-yards another, as while under way, for the purpose of checking headway or heaving to).

from the GNU version of the Collaborative International Dictionary of English.

• (Naut.) The brace of the fore-topsail on the leeward side of a vessel.
• (Engin.) A brace, in a framed structure, which resists a strain of a character opposite to that which a main brace is designed to receive.
• transitive verb (Naut.) To brace in opposite directions.
• transitive verb (Engin.) To brace in such a way that opposite strains are resisted; to apply counter braces to.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• verb nautical, transitive To brace in opposite directions.
• verb engineering, transitive To brace in such a way that opposite strains are resisted.