big gun

One that is powerful or influential: The big guns for the prosecution were its expert witnesses.

1.  Very energetically or successfully. This colloquial expression usually occurs in the phrase go great guns, as in They're going great guns with those drawings. The expression comes from British naval slang of the late 1700s, when blowing great guns meant a violent gale. Harry Truman used the term in Dear Bess (1945): "We have been going great guns in the last day or two."
2.  great gun. Also big gun. An important person, as in All the great guns came to the reception. This usage is heard less often today. [Slang; early 1800s] Also see big cheese.
3.  Great guns! An expletive expressing surprise or astonishment, as in Great guns! You're not leaving now? [Late 1800s]