bracer n.

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.