In basketball, a slam dunk is what happens when a player holding the basketball leaps into the air and forces the ball downward through the net. Among all types of scoring shots in basketball, the slam dunk is the only one that is virtually unmissable (barring some rare blunder). From this derives the metaphorical sense of slam dunk—that is, a maneuver or plan certain to succeed.
The original term for a slam dunk is dunk shot. Slam dunk was coined by LA Lakers play-by-play announcer Chick Hearn in the 1960s,1 and historical Google News searches show that it was widespread by the 70s.2 The term is usually two words, but the one-word slamdunk is fairly common on the web, and we might see it gain ground as the language’s compounding impulse takes hold.
Examples
If you thought it was a slam dunk that the law would be affirmed, you should back off of that a bit. [Washington Post]
The decision produced a quiet storm of outrage among Romney’s allies in the state, who were bewildered by the decision to make a slam-dunk race competitive … [Buzzfeed]
It would once have been a slam-dunk assignment: getting young people excited to buy cars. [NY Times Media Decoder blog]
The National Weather Service says rain that was slam-dunk certainty Tuesday will slide all the way to a mere possibility today and tonight. [Oroville Mercury-Register]
On “Law & Order,” a case of such magnitude would be a slam dunk. [Boston Herald]
References
1. http://en.wikipedia.org/wiki/Slam_dunk ↩
2. Historical Google News search of “slam dunk” ↩