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The time of youth, innocence, and inexperience, as in Back in our salad days we went anywhere at night, never thinking about whether it was safe or not. This expression, alluding to the greenness of inexperience, was probably invented by Shakespeare in Antony and Cleopatra (1:5), when Cleopatra, now enamored of Antony, speaks of her early admiration for Julius Caesar as foolish: "My salad days, when I was green in judgment, cold in blood."
American Heritage Idioms