early 14c., "piece of armor for the arms," also "thong, strap for fastening," from Old French brace "arms," also "length measured by two arms" (12c., Modern French bras "arm, power;" brasse "fathom, armful, breaststroke"), from Latin bracchia, plural of bracchium "an arm, a forearm," from Greek brakhion "an arm" (see brachio-).
The meaning "that which holds two or more things firmly together" (on the notion of clasping arms) is from mid-15c. Hence the word is applied to various devices for fastening and tightening. The meaning "a prop, support," especially in architecture, is from 1520s. Of dogs, ducks, pistols, etc., "a couple, a pair" from c. 1400.
Braces is from 1798 as "straps passing over the shoulders to hold up the trousers;" from 1945 as "wires for straightening the teeth."
mid-14c., "to seize, grasp, hold firmly," also "wrap, enshroud; tie up, fetter," from Old French bracier "to embrace," from brace "arms" (see brace (n.)). The meaning "make tense, render firm or steady by tensing" is from mid-15c., it is attested earlier in the figurative sense of "strengthen or comfort" someone (early 15c.), with a later extension to tonics, etc. that "brace" the nerves (compare bracer "stiff drink").
To brace oneself "place oneself in the position of a brace" (in anticipation of some shock or impact) is by 1805, perhaps c. 1500. To brace up "increase the tension or vigor of" is from 1809. Related: Braced; bracing.
updated on October 22, 2022