late 14c., salade, "raw herbs cut up and variously dressed," from Old French salade (14c.) and Medieval Latin salata, both from Vulgar Latin *salata, literally "salted," short for herba salata "salted vegetables" (vegetables seasoned with brine, a popular Roman dish), from fem. past participle of *salare "to salt," from Latin sal (genitive salis) "salt" (from PIE root *sal- "salt").
Dutch salade, German Salat, Swedish salat, Russian salat are from Romanic languages. Later extended to dishes composed of meat chopped and mixed with uncooked herbs and variously seasoned (chicken salad, etc.). In reference to the raw herbs and vegetables themselves, in U.S. it is colloquially limited to lettuce (by 1838).
Salad oil "olive oil used for dressing salads," is by 1550s. Salad-fork is by 1808. Salad bar is attested by 1940, American English. Salad days "time of youthful inexperience" (perhaps on notion of "green") was used by Shakespeare ("Antony and Cleopatra," 1606) and owes its survival, if not its existence, to him.
Whether the point is that youth, like salad, is raw, or that salad is highly flavoured & youth loves high flavours, or that innocent herbs are youth's food as milk is babes' & meat is men's, few of those who use the phrase could perhaps tell us ; if so, it is fitter for parrots' than for human speech. [Fowler]