Basically the principal that men are more sexually attracted and aroused to new females after a certain amount of time. The term comes from an old joke according to which President Calvin Coolidge and his wife were at a government farm one day and were taken around on separate tours. Mrs. Coolidge, passing the chicken pens, inquired of a supervisor whether the lone rooster was sufficient, given the many hens in the chicken flock.
"Yes", the man said, "the rooster works very hard."
Mrs. Coolidge then asked, "Really? The rooster works very hard? Every day?"
"Oh, yes," the man said. "Dozens of times a day."
"Interesting!" Mrs. Coolidge replied, "Be sure to tell that to the President!"
Some time later the President, passing the same pens, was told about the roosters - and about his wife's remark. "Same hen every time?", he asked.
"Oh, no, a different one each time," the supervisor replied.
"Tell that," Coolidge said with a sly nod, "to Mrs. Coolidge."
The Coolidge comes into effect in the porno industry, where 'Fluffers' are used to get male porn stars erect before a scene, if they've been with the main actress so long.
"I mean, I love her and all... But i'm just SO greedy. Sometimes, I hate the Coolidge Effect. Luckily she's bisexual."