smoking gun

from WordNet (r) 3.0 (2006)

smoking gun
    n 1: indisputable evidence (especially of a crime)

from The Collaborative International Dictionary of English v.0.48

smoking gun \smok"ing gun`\ (sm[=o]k"[i^]ng g[u^]n`), n.
   [metaphorical] [from imagined situations where a person
   holding a pistol, still smoking from having been fired, is
   found standing over a dead person]
   Incontrovertible evidence; convincing evidence.
   [PJC]