to dare is one of the meanings of unterstehen. The verb wagen is a possible synonyme. This meaning is documented since the 16th century.
Er unterstand sich, es zu tun. / Er wagte sich, es zu tun.
He dared to do it.
The tricky thing with unterstehen today is when used in imperative sentences:
Untersteh’ dich!
The same with the verb wagen:
Wag es [dir]!
Despite looking like a demand or invitation, both sentences are actually threats not to do something and have the meaning of the english Don’t you dare!
I can’t really explain why the definition changes from dare to not dare when used as an imperative, but I think of it as a non-expressed hint to consequences:
Untersteh’ dich [und du wirst sehen, was passiert]!
Wag es dir [und du wirst sehen, was passiert]!
Dare it and you’ll see what happens!
Since the imperative is the mostly used form of unterstehen I could imagine that in a few decades the meaning will completely change from dare to not dare, but that’s just pure speculation.
Other meanings of unterstehen include to accomplish/achieve which is documented since the 8th century and no longer in use today and to be under sb or to be subject to sb/sth, documented since the 17th century. Here’s an example sentence:
Ich unterstehe der Abteilung Rechnungswesen.
I’m under the accountants' section.
"Untersteh' dich"
is almost exclusively used in an ironic sense (to prevent someone from daring something) or in (mock) rebuke. Also it is slightly old-fashioned.wagen
works for both situations).