What is the explanation of how Delaware got its nickname?

Delaware's nickname "the First State" comes from its ratification of the US Constitution on December 7, 1787, the first state to do so.

Delaware's nickname "the Diamond State" is a reference to its state flag, which has its coat of arms on a large diamond-shaped field.

Delaware's nickname "the Blue Hen State" comes from the name of a variety of chicken, the Blue Hen chicken, which is the state bird.