What is old iron side?

Old Ironsides is a nickname for the USS Constitution, a wooden-hulled, three-masted heavy frigate of the United States Navy. It was launched in Boston in 1797 and is the oldest commissioned warship afloat in the world. It was named after the phrase "her sides are made of iron" in the poem "The American Constitution" by Oliver Wendell Holmes Sr. The ship gained fame during the War of 1812 when it defeated several British warships. It is now preserved as a museum ship in Boston and is a popular tourist attraction.