We took our new Japanese clients out for a large dinner last night. Because I studied Japanese for 2 years at high-school, it was decided I should make a speech in Japanese after the meal. I was very nervous about it, but fortunately the four glasses of wine I had with dinner gave me all the dutch courage (angetrunkener Mut) I needed.
The most common explanation for this idiom dates back to the Anglo-Dutch Wars of the 17th and 18th centuries. English soldiers claimed that Dutch soldiers needed to drink gin (invented by the Dutch) to give them enough courage to fight. In this sense the term is rather racist, suggesting that the Dutch would only show up for wars if they were drunk.