"tract of wet, swampy ground," 1650s, from Dutch moeras "marsh, fen," from Middle Dutch marasch, from Old French marais "marsh," from Frankish, possibly from West Germanic *marisk, from Proto-Germanic *mariskaz "like a lake," from *mari "sea" (from PIE root *mori- "body of water"). The word was influenced in Dutch by moer "moor" (see moor (n.)). Figurative use is attested from 1867. Replaced earlier mareis (early 14c.; see marish).
Proto-Indo-European root meaning "body of water."
It forms all or part of: aquamarine; Armorica; beche-de-mer; cormorant; mare (n.2) "broad, dark areas of the moon;" marina; marinate; marine; mariner; maritime; marsh; mere (n.1) "lake, pool;" Merlin; mermaid; merman; meerschaum; meerkat; morass; Muriel; rosemary; submarine; ultramarine; Weimar.
It is the hypothetical source of/evidence for its existence is provided by: Latin mare; Old Church Slavonic morje, Russian more, Lithuanian marės, Old Irish muir, Welsh mor "sea;" Old English mere "sea, ocean; lake, pool," German Meer "sea."
The distance from the mouth of Hilsborough river to the head of the lake, in a direct line, is about 110 statute miles. The country between them is mostly, if not wholly, an everglade, by which is meant a low marsh frequently covered with water, and in which there grows a sharp triangular grass, from ten to twelve feet high, and impervious to men or animals. [American Mechanics' Magazine, Jan. 21, 1826]
"tract of open, untilled, more or less elevated ground, often overrun with heath," c. 1200, from Old English mor "morass, swamp," from Proto-Germanic *mora- (source also of Old Saxon, Middle Dutch, Dutch meer "swamp," Old High German muor "swamp," also "sea," German Moor "moor," Old Norse mörr "moorland," marr "sea"), perhaps related to mere (n.1), or from root *mer- "to die," hence "dead land."
The basic sense in place names is 'marsh', a kind of low-lying wetland possibly regarded as less fertile than mersc 'marsh.' The development of the senses 'dry heathland, barren upland' is not fully accounted for but may be due to the idea of infertility. [Cambridge Dictionary of English Place-Names]
Hence moor-fowl "grouse" (c. 1500); moor-hen (mid-14c.); moor-cock (c. 1200 as a surname).
c. 1500 (implied in swamwatyr "swamp-water"), of uncertain origin, perhaps [Barnhart] a dialectal survival from an Old English cognate of Old Norse svöppr "sponge, fungus," from Proto-Germanic *swampuz; but traditionally connected with Middle English sompe "morass, swamp," which probably is from Middle Dutch somp or Middle Low German sump "swamp" (see sump). All of these likely are ultimately related to each other, from PIE *swombho- "spongy; mushroom," via the notion of "spongy ground."
[B]y swamps then in general is to be understood any low grounds subject to inundations, distinguished from marshes, in having a large growth of timber, and much underwood, canes, reeds, wythes, vines, briers, and such like, so matted together, that they are in a great measure impenetrable to man or beast .... [Bernard Romans, "A Concise History of East and West Florida," 1775]
More popular in U.S. (swamp (n.) by itself is first attested 1624 in Capt. John Smith's description of Virginia). Swamp-oak is from 1680s, American English. Swamp Yankee "rural, rustic New Englander" is attested from 1941. Thornton's "American Glossary" (1912) has swamp-angel "dweller in a swamp," swamp-law "might makes right."