late 14c., from Old French salade (14c.), 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. Salad days "time of youthful inexperience" (perhaps on notion of "green") is first recorded 1606 in Shakespeare and probably owes its survival, if not its existence, to him. Salad bar first attested 1940, American English.