brace (n.)

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.