"large, heavy plate or tray on which anything is presented," 1660s, formed in English on the model of platter, etc., from French salve "tray used for presenting objects to the king" (17c.), from Spanish salva "a foretasting of the food or drink" of one's master, to test it for poison (a procedure known as pre-gustation). Hence "tray on which food was placed to show it was safe to eat." The Spanish noun is from salvar "to save, render safe," from Late Latin salvare (see save (v.)).
Compare credenza, which means etymologically "belief" and began as the word for a sideboard on which taste-tested food was set. Middle English had salver in the sense of "a healer," used as an epithet of Jesus or the Virgin.