The inclination to become unfaithful after seven years of marriage.
Remember Marilyn Monroe's dress blowing in the wind over a subway grating? One of the most enduring images from Hollywood, the scene was from the movie, The seven-year itch. The expression was used to indicate the urge for infidelity after seven years of marriage. Though today it has gained wider acceptance in terms of its scope: it now refers to an urge to move on from any existing situation, and not even limited to those of seven years' period.
The original seven-year itch wasn't a condition that supposedly began after seven years, but one that supposedly lasted for seven years. Seven-year itch had been known in the USA since the early 19th century as the name of a particularly irritating and contagious skin complaint (don't ask me which one, coz despite thorough search I could not find the answer to that: it could be scabies or poison-ivy itch (very unlikely though) ) that led to highly irritating red pimples on the face and body.
The term was virtually forgotten after the cure for the complaint was found till the 1955 film revived it.
Source: http://en.wikipedia.org/ , http://www.phrases.org.uk/,
Update: (05/05/07) by 666
The follow-up research to Moi’s post was incredibly fascinating. I came across numerous citations (mentioned below) explaining some sort of link between ‘seven year itch’ and scabies.
The 'seven year itch' has its origins with a microbe known as Sarcoptes scabiei, more commonly called 'scabies.' The bug produces an itching skin irritation that before modern drugs lasted, on average seven years.
"Why seven years, not six or eight? Because seven years has a historical basis: In Genesis, Joseph interprets Pharaoh's dream of 'seven years of great plenty' followed by 'seven years of famine.'." And so forth. – (1)
However, the medico’s have a slightly different version. Scabies is known as "The Seven Year Itch," because its incidence rises and falls as regularly as the tides, or the sunspots, turning up in abundance every seven years. – (2)
Read in detail about scabies here and here.
(1) - http://www.phrases.org.uk
(2) - http://www.ralphmag.org/DI/scabies1.html