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.