The Horatio Alger thought experiment thing-
you are a struggling inventor, on your way to a fob-off meeting with the assistant deputy paper-clip manager in a big company that could make you rich as you make them rich. You step into the elevator and… holy shit, the only other person in it is the guy who founded the company and now weighs rather than counts his take-home pay. You have 45 seconds to explain what his problem is, what your solution is, and what your track record in delivering what you promise is…. You’re not aiming for a sale, just to pique enough interest so you talk to an organ-grinder rather than a monkey…
Or, as wikipedia would have it-
The name ‘elevator pitch’ reflects the idea that it should be possible to deliver the summary in the time span of an elevator ride, or approximately thirty seconds to two minutes and is widely credited to Ilene Rosenzweig and Michael Caruso (while he was editor for Vanity Fair) for its origin. The term itself comes from a scenario of an accidental meeting with someone important in the elevator. If the conversation inside the elevator in those few seconds is interesting and value adding, the conversation will either continue after the elevator ride, or end in exchange of business cards or a scheduled meeting. A variety of people, including project managers, salespeople,evangelists, and policy-makers, commonly rehearse and use elevator pitches to get their points across quickly.