The term 'secret weapon' is used to either describe a weapon that is concealed from another, or a weapon that has not been confirmed by its user. It could also come to describe a person which someone believes will come to their aid, or defense, or help them archive something which the public or individual have no idea about. In the terms of a large scale weapons, it is also a newly developed secret weapon created by a government such as the Atomic Bomb created during the 1940's by the United States of America.
As a theme it has been a frequent subject in popular culture in comics. It has been used to add an element of suspense such as when a hero is either attempting to stop the use of an arch enemy's secret weapon or having an ace in the hole when it comes to a fight.