This figurative expression for days of inexperienced youth or sometimes better days is pure Shakespeare. They are Cleopatra’s words in Antony and Cleopatra, Act I, Scene V. “My salad days, when I was green in judgment - cold in blood.” The operative word here is green as in not ripe or immature. Incidentally, a salad in Shakespeare’s days would have been a more substantial dish than just a few green leaves. It would have been a cold dish of mixed vegetables, hard-boiled eggs, fish, meat etc. This would be seasoned with salt from which the word salad derives, from the Latin sal, salt. Salami, the salted Italian salted sausage, comes from the same root. See also salary.