"tract of water-soaked or partially flooded land; wet, swampy ground; piece of low ground, usually more or less wet but often nearly dry at certain seasons," Middle English mersh, from Old English mersc, merisc "marsh, swamp," from Proto-Germanic *marisko (source also of Old Frisian and Old Saxon marsk "marsh," Middle Dutch mersch, Dutch mars, German Marsch, Danish marsk), probably from Proto-Germanic *mari- "sea" (from PIE root *mori- "body of water").
The vowel shift from -e- to -a- began in 15c. and is usual for -er- followed by a consonant: Compare darling (Middle English dereling, Old English deorling), far (Middle English fer, Old English feorr), mar (Middle English merren), hart (Middle English hert, Old English heorot). Marsh gas "methane generated by decaying matter in marshes" is attested by 1819.
Middle English salten, "prepare with salt, preserve (something) with salt," from Old English sealtan, from Proto-Germanic *salto- (see salt (n.)), and in part from the noun. Related: Salted; salting.
This is reconstructed to be from Proto-Italic *atno- "year" (compare Oscan akno- "year, holiday, time of offering"), from PIE *at-no- "which goes," also "a year" (as "going around"), suffixed form of root *at- "to go" (source also of Sanskrit atati "goes, wanders," atamana- "to travel, wander," atya- "steed, runner"). The root also has Germanic derivatives meaning "a year," such as Gothic aþnam (dative plural) "year."
Old English sealt "salt, sodium chloride, abundant substance essential to life, used as a condiment and meat preservative," from Proto-Germanic *saltom (source also of Old Saxon, Old Norse, Old Frisian, Gothic salt, Dutch zout, German Salz), from PIE root *sal- "salt."
Applied from early 14c. to various substances resembling common salt. Modern chemistry sense "compound of an acid radical with a base radical" is from 1790; as an ultimate element in alchemy from 1580s. Meaning "experienced sailor" is attested by 1840 (Dana), probably a reference to the salinity of the sea. By 1570s as "that which gives piquancy to discourse or writing or liveliness to a person's character."
Salt long was 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 "efficient, capable" (1830), salt of the earth "persons of worthiness" (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-shaker is from 1882. Salt-and-pepper (adj.) "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 "accept with a certain amount of reserve" is from 1640s, from Modern Latin cum grano salis. The notion is perhaps "modification," hence "allowance, abatement, reserve."
Old English sealt, "salty, briny, containing salt," from Proto-Germanic *saltoz-, from the source of salt (n.). By c. 1300 as "treated with or preserved with salt" (salt fish).
Cold War U.S.-U.S.S.R. nuclear weapons negotiations, 1968, acronym for Strategic Arms Limitation Talks (which would make SALT talks redundant, but the last element sometimes also is understood, especially after their outcome, as treaty).