What's a "brace" in the expression "brace yourself"? [closed]

I know the meaning of the expression, "brace yourself," and also the meaning of the word "brace" but I don't understand why they have that word in that expression and what its origin or history is.

Why brace? Aren't braces what dentists install in people's jaws?

Brace in brace yourself means prepare yourself for something difficult or unpleasant. This is a subsense of the verb meaning make a structure stronger or firmer.

I think brace in this sentence is meant as a definition that has not been mentioned here yet. It can mean according to the Oxford dictionary to:

Press (one’s body or part of one’s body) firmly against something in order to stay balanced.

eg: "He braced his shaking body against the wall and forced his legs to defy gravity and support him one last time."

I have always thought it was this meaning used here as it is usually said when one is supposed to literally grab or press against something to prevent themselves from falling or being injured.

Brace has different meanings, apart from the one you mention:

1) A device that holds or fastens two or more parts together or in place; a clamp.

2) A cause or source of renewed physical or spiritual vigor.
Source: Collins Dict.

Definition 2 is obviously a figurative sense of definition 1, which by extension is used in the saying brace oneself, meaning: prepare oneself (with adequate psychological support) to face difficult events.

Brace is from the French bras meaning arms. It probably arrived with the Normans. The command Brace! - still used in aviation - originaly meant "hold on with your arms" (otherwise you'll be flung out of the longboat as it hits the beach!) By extension, a brace serves to hold things in place.

A dentist attaches a brace to teeth not the jaw.