Dutch courage (see also: potvaliancy) refers to courage gained from intoxication with alcohol. It may also be used as a synonym for gin.
The popular story dates the etymology of the term to English soldiers fighting in the Thirty Years' War. One version states that Dutch gin was used by English soldiers for its believed warming properties on the body in cold weather and its calming effects before battle; another version states that English soldiers noted the bravery-inducing effects of jenever's liquor on Dutch soldiers and dubbed it "Dutch Courage". Gin would go on to become popular in England thanks to King William III, who was also Stadtholder of the Netherlands, better known as William of Orange (1689-1702).
References
Dutch courage Wikipedia(Text) CC BY-SA