Salad days are the days of youth, when one is green and fresh. It was coined by Shakespeare in his 1606 Antony & Cleopatra, I.v.73:
Sallad dayes, When I was greene in iudgement, cold in blood.
Shakespeare’s use pretty much stood alone until the mid-19th century, when the term began to be used as a literary allusion. From The Cornhill Magazine of May 1865:
Being in want of a horse at the time—it was in my salad days, reader—I looked through the advertisements in The Times, and noticed one which at any rate promised well.
(Source: Oxford English Dictionary, 2nd Edition)
Copyright 1997-2016, by David Wilton