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.