Countries that currently, or in recent history have depended upon the United States. Often used in an arrogant manner by some Americans who think that somehow showing support towards a nation should somehow give them a stake in that nation. Also often used by people noting that the culture of a country seems, or that a country shows a lot of support towards the US, or because of their cultural similarity to the US.
First usage: The United Kingdom due to the assistance from Americans in World War II, is sometimes a considered a 51st state.
Second usage: The United Kingdom is considered a 51st state due to their government's support of America in the Iraq war.
What some people call Canada. Actually, Canada is another country; the twin country of the United States of America, north of it. We don't live in igloos, either.
The use of the word "blue" to refer to sexually explicit content was first recorded in Scotland in 1824. "Blue" meant "lewd" because prostitutes dressed in blue gowns.