A difficult problem; a hard person to deal with. This early analogy, also put as a hard nut to crack, was first drawn in the early eighteenth century. Benjamin Franklin used it in a letter in 1745: “Fortified towns are hard nuts to crack; and your teeth have not been accustomed to it.” A similar term from a somewhat later era is tough customer, likewise meaning a person difficult to deal with. Dickens used it in Barnaby Rudge (1841): “Rather a tough customer in argument, Joe, if anybody was to try and tackle him.”