Days of inexperience, when persons are very green.
My salad days. When I was green in judgment.
Shakespeare: Antony and Cleopatra, i. 5.
A pen'orth of salad oil. A strapping; a castigation. It is a joke on All Fools' Day to send one to the saddler's for a “penorth of salad oil.” The pun is between “salad oil,” as above, and the French avoir de la salade, “to be flogged.” The French salader and salade are derived from the salle or saddle on which schoolboys were at one time birched. A block for the purpose used to be kept in some of our public schools. Oudin translates the phrase “Donner la salle à un escolier” by “Scopar un scolari innanzi à tutti gli altri.” (Recherches Italiennes et Francoises, part ii. 508.)
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894