mountain in Caernarvonshire, northern Wales, from English snow (n.) + Old English dun "hill, mountain" (see down (n.2); presumably translating a former Celtic name. The height is snow-covered much of the year.
Old English snaw "snow, that which falls as snow; a fall of snow; a snowstorm," from Proto-Germanic *snaiwaz (source also of Old Saxon and Old High German sneo, Old Frisian and Middle Low German sne, Middle Dutch snee, Dutch sneeuw, German Schnee, Old Norse snjor, Gothic snaiws "snow"), from PIE root *sniegwh- "snow; to snow" (source also of Greek nipha, Latin nix (genitive nivis), Old Irish snechta, Irish sneachd, Welsh nyf, Lithuanian sniegas, Old Prussian snaygis, Old Church Slavonic snegu, Russian snieg', Slovak sneh "snow"). The cognate in Sanskrit, snihyati, came to mean "he gets wet." As slang for "cocaine" it is attested from 1914.
"a hill of moderate elevation and more or less rounded outline," Old English dun "height, hill, moor," from Proto-Germanic *dunaz- (source also of Middle Dutch dunen "sandy hill," Dutch duin), "probably a pre-insular loan-word from Celtic" [Cambridge Dictionary of English Place-Names], in other words, borrowed at a very early period, before the Anglo-Saxon migration, perhaps from PIE root *dheue- "to close, finish, come full circle."
The more general meaning "elevated rolling grassland; high, rolling region not covered by forest" is from c. 1400. Specifically of certain natural pastureland districts of south and southeast England (the Downs) by mid-15c.
The non-English Germanic words tend to mean "dune, sand bank" (see dune), while the Celtic cognates tend to mean "hill, citadel" (compare Old Irish dun "hill, hill fort;" Welsh din "fortress, hill fort;" and second element in place names London, Verdun, etc.). German Düne, French dune, Italian, Spanish duna are said to be loan-words from Dutch.