I am (one is) extremely eager to do something. The use of “itch” for “desire” is almost as old as the desire to scratch something that itches. “Our fingers wyll itch at hym,” wrote John Stubbs (The Discoverie of a Gaping Gulf, 1579), and soon afterward Shakespeare wrote, “If I see a word out, my finger itches to make one” (The Merry Wives of Windsor, 1601, 2.3). Considerably later came Charles Kingsley with his “The men’s fingers are itching for a fight” (Hypatia, 1853), which subsequently was shortened to itching for a fight. See also itchy palm.

A yearning for change, specifically a change of sexual partner after seven years of marriage. This expression gained currency through George Axelrod’s play The Seven-Year Itch (1952), later made into a movie starring Marilyn Monroe (1955). Originally the phrase was used for various skin conditions. One such use was for poison ivy, in the belief that the rash would recur every year for seven years. Also, centuries ago itch was a slang word for “sexual desire.” Today, the expression invariably refers to the wish for a new sexual partner and, for some reason, tends to be applied only to men. Thus Patricia Moyes wrote, “There’s something called the seven-year itch . . . middle-aged men quite suddenly cutting loose” (Angel of Death, 1980).