Bockscar, sometimes called Bock's Car, is the name of the United States Army Air Forces B-29 bomber that dropped the "Fat Man" nuclear weapon over Nagasaki on 9 August 1945, the second atomic weapon used against Japan. It was assigned to the 393d Bomb Squadron, 509th Composite Group.
The name painted on the aircraft after the mission is a pun on "boxcar" after the name of its aircraft commander, Captain Frederick C. Bock.
Read more about Bockscar: Atomic Bomb Mission, Airplane History, Current Status, National Museum of The United States Air Force Display