Pero...
George Axelrod, who wrote the play on which the film was based, didn't coin the phrase though. It came from an earlier US source. 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. The name was well enough known by 1845 for the condition to be used as a metaphor for all that is annoying; for example, this item from that year, from the Wisconsin Herald and Grant County Advertiser:
