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-).
Meaning "that which holds two or more things firmly together" (on notion of clasping arms) is from mid-15c. Hence applied to various devices for fastening and tightening. 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.)). Meaning "make tense, render firm or steady by tensing" is mid-15c., earlier in figurative sense "strengthen or comfort" (someone), early 15c., with 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.