Old English sealt "salt" (n.; also as an adjective, "salty, briny"), from Proto-Germanic *saltom (source also of Old Saxon, Old Norse, Old Frisian, Gothic salt, Dutch zout, German Salz), from PIE root *sal- "salt."
Modern chemistry sense is from 1790. Meaning "experienced sailor" is first attested 1840, in reference to the salinity of the sea. Salt was long regarded as having power to repel spiritual and magical evil.
Many metaphoric uses reflect that this was once a rare and important resource, such as worth one's salt (1830), salt of the earth (Old English, after Matthew v.13). Belief that spilling salt brings bad luck is attested from 16c. To be above (or below) the salt (1590s) refers to customs of seating at a long table according to rank or honor, and placing a large salt-cellar in the middle of the dining table.
Salt-lick is recorded by 1751; salt-marsh is Old English sealtne mersc; salt-shaker is from 1882. Salt-and-pepper "of dark and light color" is by 1915 (pepper-and-salt, 1774, was an old name for a kind of cloth made from dark and light colored wools woven together). To take something with a grain of salt is from 1640s, from Modern Latin cum grano salis.