Facts on File Encyclopedia of Word and Phrase Origins
SEVEN-YEAR ITCH: Used to describe a husband’s or wife’s urge to stray from his or her mate after seven years of marriage, this expression appears to have been invented by American playwright George Axelrod I his play ‘T he Seven year Itch (1952) and further popularized by the film starring Marilyn Monroe (1955). Word and phrase hunters haven’t been able to turn up any earlier use of the words in a sexual context, although ‘seven-year itch’ had been used to describe a poison ivy itch that supposedly recurred once every seven years.
Random House unabridged Dictionary
SEVEN-YEAR ITCH 1. scabies [a contagious skin disease occurring esp. in sheep and cattle and also in humans, caused by the itch mite, ‘Sarcoptes scabiei,’
which burrows under the skin.]. 2. the temptation to have an affair, held to become a hazard in the seventh year of marriage.[1895–1900, American].
[[Not clear here which definition this date refers to. Notice that Facts on File and Merriam-Webster seem to disagree on the nature of the ‘skin problem.’]]
Ken G – October 17, 2002