hydrocarbon radical occurring in many compounds, 1835, from French méthylène (1834), coined by Jean-Baptiste-André Dumas (1800-1884) and Eugène-Melchior Péligot (1811-1890) from Greek methy "wine" (see mead (n.1)) + hylē "wood" (which is of uncertain etymology) + Greek name-forming element -ene. So called because it was detected in wood alcohol.
"The breakdown of methylene into methyl and -ene, and the identification of the last syllable of methyl with the general suffix -yl, led to the use of meth- as a separate combining-element, as, for example, in methane, methacrylic" [Flood]. The color methylene-blue (1880) was derived from dimethylanaline.
"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.