Online Etymology Dictionary

dew (n.): Old English deaw, from Proto-Germanic *dawwaz (source also of Old Saxon dau, Old Frisian daw, Middle Dutch dau, Old High German tau, German Tau, Old Norse dögg "dew"), from PIE root *dheu- (2) "to flow" (source also of Sanskrit dhavate "flows, runs").
dew claw (n.): also dew-claw, 1570s, from claw, but the signification of the first element is obscure (see dewlap).
dewpoint (n.): 1833; see dew + point (n.).
dewy (adj.): Old English deawig (see dew + -y (2)).
dewdrop (n.): early 14c. (as two words); see dew + drop (n.).
honeydew (n.): also honey-dew, 1570s, "sticky sweet substance found in small drops on trees and plants," from honey (n.) + dew (n.); Similar formation in Dutch honigdaauw, German Honigthau. honeydew melon first recorded 1916, a cross between cantaloupe and a South African melon.
dag (n.): "thin rain, drizzle, wet fog," late 17c., from a Scandinavian source such as Old Norse dögg, plural daggir "dew," from Proto-Germanic *daowo- (source of Old English deaw; see dew).
mildew (n.): mid-13c., mildeu "honeydew, nectar," from Old English meledeaw "honeydew" (sticky stuff exuded by aphids), from Proto-Germanic compound of *melith "honey" (see Melissa) + *dawwaz "dew" (see dew). Similar formation in Old Saxon milidou, Dutch meeldauw, German Meltau "mildew."

First element in many cases assimilated to forms of meal (n.2) "ground grain." As a kind of fungus it is first recorded mid-14c., so called from its being sticky and originally growing in plants. As a verb from 1550s. Related: Mildewed.
mountain (n.): c. 1200, from Old French montaigne (Modern French montagne), from Vulgar Latin *montanea "mountain, mountain region," noun use of fem. of *montaneus "of a mountain, mountainous," from Latin montanus "mountainous, of mountains," from mons (genitive montis) "mountain" (see mount (n.1)).

Until 18c., applied to a fairly low elevation if it was prominent (such as Sussex Downs, the hills around Paris). As an adjective from late 14c. Mountain dew "raw and inferior whiskey" first recorded 1839; earlier a type of Scotch whiskey (1816); Jamieson's 1825 "Supplement" to his Scottish dictionary defines it specifically as "A cant term for Highland whisky that has paid no duty." Mountain-climber recorded from 1839; mountain-climbing from 1836.
Dewey Decimal system: proposed 1876 by Melvil Dewey (1851-1931) while acting librarian of Amherst College. He also crusaded for simplified spelling and the metric system.
dewlap (n.): mid-14c., dewe lappe, from lappe "loose piece" (Old English læppa), first element of unknown origin or meaning. Originally of cattle.
hoarfrost (n.): "white frost formed by freezing dew," c. 1300, hore-forst; see hoar + frost (n.).
drosophila (n.): scientific name of a fruit fly, 1829, from Modern Latin (Fallén, 1823), from Greek drosos "dew" + philos "loving" see -phile).
rosemary (n.): late 14c., earlier rosmarine (c. 1300), from Latin rosmarinus, literally "dew of the sea" (compare French romarin), from ros "dew" + marinus (see marine (adj.)). Perhaps so called because it grew near coasts. Form altered in English by influence of rose and Mary.

Latin ros is from PIE *ers- "to be wet" (source also of Lithuanian rasa, Old Church Slavonic rosa "dew," Sanskrit rasah "sap, juice, fluid, essence," Hittite arszi "flows," and perhaps also Rha, Scythian name of the River Volga (see rhubarb)).
drizzle (v.): 1540s, perhaps an alteration of drysning "a falling of dew" (c. 1400), from Old English -drysnian, related to dreosan "to fall," from PIE root *dhreu- (see drip (v.)). Or perhaps a frequentative of Middle English dresen "to fall," from Old English dreosan. Related: Drizzled; drizzling. As a noun, from 1550s.
precipitation (n.): late 15c., "a casting down" (of the evil angels from heaven), also, in alchemy "separation of a solid substance from a solution," from Middle French precipitation (15c.) and directly from Latin praecipitationem (nominative praecipitatio) "act or fact of falling headlong, haste," noun of action from past participle stem of praecipitare "fall, be hasty," from praeceps "steep" (see precipice). Meaning "sudden haste" is c. 1500. Meaning "act of falling from a height" is attested from 1610s. Meteorological sense of "rain, snow, dew, etc." is from 1670s.
frost (n.): Old English forst, frost "frost, a freezing, frozen precipitation, extreme cold," from Proto-Germanic *frustaz- "frost" (source also of Old Saxon, Old Frisian, Old High German frost, Middle Dutch and Dutch vorst), related to freosan "to freeze," from suffixed form of PIE *preus- "to freeze; burn" (see freeze (v.)). Both forms of the word were common in English till late 15c.; the triumph of frost may be due to its similarity to the forms in other Germanic languages. A black frost (late 14c.) is one which kills plants (turns them black) but is not accompanied by visible frozen dew.
micturition (n.): 1725, "the need very badly to urinate," from Latin micturitum, from past participle of micturire "to desire to urinate," desiderative of mingere "to urinate," from PIE *meigh- "to urinate" (source also of Sanskrit mehati "urinates;" Avestan maezaiti "urinates;" Greek omeikhein "to urinate;" Armenian mizem "urinate;" Lithuanian minžu "urinate;" Old English migan "to urinate," micga "urine," meox "dung, filth"). As during the final 20 minutes of a 4-hour film after drinking a 32-ounce Mountain Dew from the snack bar and the movie ends with a drawn-out farewell scene while Frodo is standing on the pier and wavelets lap audibly on the dock the whole time as if the director was a sadist set on compounding your torment.
lunch (n.): "mid-day repast," 1786, shortened form of luncheon (q.v.). The verb meaning "to take to lunch" (said to be from the noun) also is attested from 1786:
PRATTLE. I always to be ſure, makes a point to keep up the dignity of the family I lives in. Wou'd you take a more ſolid refreſhment?--Have you lunch'd, Mr. Bribe?

BRIBE. Lunch'd O dear! Permit me, my dear Mrs. Prattle, to refreſh my sponge, upon the honey dew that clings to your raviſhing pouters. O! Mrs. Prattle, this ſhall be my lunch. (kiſſes)

["The Mode," in William Davies' "Plays Written for a Private Theatre," London, 1786]
But as late as 1817 the only definition of lunch in Webster's is "a large piece of food." OED says in 1820s the word "was regarded either as a vulgarism, or as a fashionable affectation." Related: Lunched; lunching. Lunch money is attested from 1868; lunch-time (n.) is from 1821; lunch hour is from 1840. Slang phrase out to lunch "insane, stupid, clueless" first recorded 1955, on notion of being "not there." Old English had nonmete "afternoon meal," literally "noon-meat."