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:
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.