... when one asked what it meant, the answer was:

The amount of time it takes when a traffic light turns green and the car behind you blows its horn [a split second].

I guess the saying has changed thru the years like the meaning of the term "deep pockets" which meant in the early-60s: A person who was cheap and never came up with the cash because his pockets were so deep he couldn't reach down to grab his money.