What does "brace up" mean?

Also, brace oneself. Summon up one's courage or resolve, as in Brace up, we don't have much farther to go, or Squaring his shoulders, he braced himself for the next wave. This idiom uses brace in the sense of “to bolster” or “to strengthen.” The first term dates from the early 1700s, the variant from about 1500.