What does "salad days" mean, and when was it coined?
Solution 1:
It refers to the time of youth when one was naive and inexperienced, and therefore happy and optimistic - in other words, when one was "green," as in "unripe" or "not yet mature." It's a bit of a convoluted pun.
Like so many English idioms, the term was coined by Shakespeare in the 17th century (Antony and Cleopatra.)
Solution 2:
Just agreeing with @PyroTyger, with the actual quote:
Charmian: By your most gracious pardon, I sing but after you.
Cleopatra: My salad days, When I was green in judgment, cold in blood, To say as I said then!