Etymology
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mildew (n.)

"minute parasitic fungus that appears on plants or decaying organic matter," mid-14c., a transferred sense of a word that meant originally "nectar, honeydew" (the sugar-rich sticky stuff secreted by aphids feeding on plant sap); this is from mid-13c. as mildeu, from Old English meledeaw, from a Proto-Germanic compound of *melith "honey" (from PIE root *melit- "honey") + *dawwaz "dew" (see dew). Similar formation in Old Saxon milidou, Dutch meeldauw, Old High German miltou, German Meltau "mildew." The first element in many continental Germanic languages has been assimilated to forms of meal (n.2) "ground grain."

As a kind of morbid fungus or blight, it presumably is so called from its being sticky and growing on plants. As a verb, "to taint with mildew," from 1550s. Related: Mildewed; mildewy.

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*melit- 

Proto-Indo-European root meaning "honey."

It forms all or part of: caramel; marmalade; Melissa; mellifluous; mildew; molasses; mousse.

It is the hypothetical source of/evidence for its existence is provided by: Greek meli, Latin mel "honey; sweetness;" Albanian mjal' "honey;" Old Irish mil "honey," Irish milis "sweet;" Old English mildeaw "nectar," milisc "honeyed, sweet;" Old High German milsken "to sweeten;" Gothic miliþ "honey."

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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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