(capitalized, informal, often, uncountable, usually) The atomic bomb; the capability to launch a nuclear attack.
(slang, uncountable, usually) A success; something excellent.
Etymology
Attested in reference to nuclear weapons or the capacity to use them since 1932; see quotations, below. As slang for something excellent, the phrase is attested since the 1960s; compare go down a bomb, UK theatre slang during the 1950s meaning “be a major success”. For further etymology see bomb.