Etymology
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moss (n.)

the meanings "mass of small, cryptogamous, herbaceous plants growing together" and "bog, peat-bog" are the same word: Old English meos "moss plant" and mos "bog;" both are from Proto-Germanic *musan (source also of Old High German mios, Danish mos, German Moos), also in part from Old Norse mosi "moss, bog," and Medieval Latin mossa "moss," from the same Germanic source.

These are from PIE *meus- "damp," with derivatives referring to swamps and swamp vegetation (source also of Latin muscus "moss," Lithuanian mūsai "mold, mildew," Old Church Slavonic muchu "moss"). The Germanic languages have the word in both senses, which is natural because moss is the characteristic plant of boggy places. It is impossible to say which sense is original. The proverb that a rolling stone gathers no moss is suggested from 14c.:

Selden Moseþ þe Marbelston þat men ofte treden. ["Piers Plowman," 1362]

Moss-agate "agate stone with moss-like dendrite forms (caused by metallic oxides)" is from 1790. Scott (1805) revived 17c. moss-trooper "freebooter infesting Scottish border marshes" (compare bog-trotter).

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club-moss (n.)

1590s, from club (n.) + moss. So called for the club shape of its upright spore-cases.

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mossy (adj.)

early 15c., "like moss, downy, velvety, or hairy;" 1560s, "overgrown with moss," from moss + -y (2).

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mysophobia (n.)

"morbid fear of contamination, dread of dirt or defilement," 1879, from Greek mysos "uncleanliness," which is of uncertain origin; perhaps from PIE *meus- "damp" (see moss) + -phobia.

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mire (n.)

"deep mud, bog, marsh, swampland," c. 1300, from a Scandinavian source such as Old Norse myrr "bog, swamp," from Proto-Germanic *miuzja- (source of Old English mos "bog, marsh"), from PIE *meus- "damp" (see moss).

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mossback (n.)

"extreme conservative, one attached to antiquated notions," 1874, American English, used especially of poor rural whites; earlier (1872) in reference to those from the Carolinas who had hid out to avoid service in the Confederate army (and would have stayed out "till the moss grew on their backs"); from moss + back (n.). The same image is behind the use of the word in angling for "a large old bass or other fish" (by 1889).

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must (n.1)
"new wine," Old English must, from Latin mustum (also source of Old High German, German most, Old French moust, Modern French moût, Spanish, Italian mosto), short for vinum mustum "fresh wine," neuter of mustus "fresh, new, newborn," perhaps literally "wet," and from PIE *mus-to-, from root *meus- "damp" (see moss).
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litmus (n.)
"blue dye-stuff obtained from certain lichens," early 14c., lit-mose, probably from an Old Norse word related to Norwegian dialectal litmose, from Old Norse lita "to dye, to stain" (from litr "color, dye;" see lit (n.1)) + mos "moss." Said to be also in part from Middle Dutch lijkmoes (Dutch lakmoes), from lac (see lac) + moes "pulp." Another idea [Watkins] connects the first element to Middle Dutch leken "to drip, leak" (see leak (v.)). The second element is in any case the common Germanic word for "moss, lichen" (see moss).

The dye is obtained from certain lichens. It is naturally blue but turns red in acid and is restored to blue by alkalis. Figurative use of litmus test is first attested 1957, from scientific use of litmus-treated paper as a chemical indicator. Litmus paper with this meaning is from 1803.
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bryo- 
word-forming element meaning "moss" in scientific compounds, from Greek bryos, bryon "moss."
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sphagnum (n.)
genus of mosses, peat-moss, 1741, Modern Latin, from Latin sphagnos, a kind of lichen, from Greek sphagnos "a spiny shrub, a kind of moss," of unknown origin. Related: Sphagnous.
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