"Dutch Treat", "I'm a Dutchman" and "Dutch courage" come from the 17th century when the Dutch were hated military and commercial rivals of the English.
Examples: "Dutch reckoning" - a bill that is presented without any details, and which only gets bigger if you question it;
"Dutch widow" - a prostitute;
"Dutch auction" - one in which the prices go down instead of up;
"Dutch courage: - temporary bravery induced by alcohol;
"Dutch metal" - an alloy of copper and zinc used as a substitute for gold foil;
"Dutch comfort" or "Dutch consolation" - where somebody might say "thank God it is no worse!";
"Dutch concert" - each musician plays a different tune;
"Dutch uncle" - someone who criticises or rebukes you with the frankness of a relative.