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The expression mountain dew was originally used in literature to designate dew of the kind that settles on mountains.
The earliest occurrence that I have found of this usage of the expression mountain dew is from the following poem, dated Sunday 10th March 1745, by ‘Z. Z.’, published in The London Magazine: Or, Gentleman’s Monthly Intelligencer (London, England) of March 1747 [page 140, column 1]:
Hymn to Aurora.
Arise, bright goddess of the ruddy morn!
Forth from your oozy bed; direct your way
Through humid traces, grey-ey’d gleams,
Misty mazes, azure streams,
By foaming coursers whirl’d in gilded car,
To paths etherial.
There, loose array’d
In flowing mantle of a saffron hue,
The gladsom East with radiance all adorn;
And skim your passage o’er the mountain dew,
In swift pursuit after your guiding star,
The morning harbinger.
The expression mountain dew came to be also used, colloquially, to designate whisky or other spirit, especially when home-made or illicit.
—Cf. the noun moonshine.
The use of the noun dew in the expression mountain dew, designating whisky or other spirit, is similar to its use in the expression Bacchus’ dew, designating the juice of the grape, i.e., wine, in the following from A Myrroure for Magistrates. Wherein may be seen by example of other, with howe greuous plages vices are punished: and howe frayle and vnstable worldly prosperitie is founde, even of those, whom fortune seemeth most highly to fauour (Londini: In ædibus Thomæ Marshe, [1559]), compiled by the English author and printer William Baldwin [Hovv George Plantagenet third sonne of the Duke of Yorke, vvas by his brother King Edvvard vvrongfully imprisoned, and by his brother Richard miserably murdered, page unnumbered]:
What prince I am although I nede not shewe.
Because my wine bewrayes me by the smell,
For never was creature sowst in Bacchus dewe
To death but I.
The expression mountain dew, designating whisky, occurs, for example, in the following from Three cheers for Scotland’s water of life, by Claire Walker, published in The Press and Journal (Aberdeen, Aberdeenshire, Scotland) of Monday 14th January 2002 [page 14, column 3]:
Whisky is such an inherent part of our culture that a whole lexicon has grown around it. You can’t say that for vodka, can you?
The only nickname for that particular tipple is voddie, as in: “Make mine a wee voddie.”
For whisky, on the other hand, we have dram, drappie, barley bree, juice o the barley, mountain dew, wee goldie, John Barleycorn, richt guid-willie waught, wee sensation, blue, snifter and hooch.
You can probably add to the list.
These are, in chronological order, the earliest occurrences that I have found of the expression mountain dew used to designate whisky or other spirit:
—Note: It seems that the original reference was to whisky distilled in the Highlands, i.e., in the mountainous region of Scotland:
1-: From Tales of my Landlord, collected and arranged by Jedediah Cleishbotham, schoolmaster and parish-clerk of Gandercleugh (Edinburgh: Printed for William Blackwood, 1816), by the Scottish poet and novelist Walter Scott (1771-1832) [Volume 1, Introduction, page 12]:
The Exciseman pretended, that my deceased Landlord did encourage that species of manufacture called distillation, without having an especial permission from the Great, technically called a licence, for doing so. Now I stand up to confront this falsehood; and, in defiance of him, his gauging-stick, and pen and inkhorn, I tell him, that I never saw, or tasted, a glass of unlawful aqua vitæ in the house of my Landlord; nay, that, on the contrary, we needed not such devices in respect of a pleasing and somewhat seductive liquor, which was vended and consumed at the Wallace Inn, under the name of mountain dew.
2-: From the Perth Courier (Perth, Perthshire, Scotland) of Thursday 22nd January 1818 [page 4, column 3]:
We understand that General Campbell of Monzie, considering the severe pressure of the times, has reduced the rents of his estate 25 per cent. In consequence of this, all his tenants, pendiclers, and cottagers, unanimously met in the village of Monzie, to express their gratitude to their benevolent patron. A bon-fire was immediately kindled; the windows illuminated; all the music which this ancient village afforded instantly struck up. The health of General Campbell was drunk in bumpers of the real mountain dew, and crowned with a burst of cheers, expressive of the genuine feelings of the peasantry. Joy smiled on every face,—gratitude glowed in every bosom; the youth were all sprightliness and mirth, and even the aged seemed to forget that they were old. The conduct of this benevolent gentleman cannot be sufficiently extolled, nor in these times can it be too often imitated.
3-: From an account of the anniversary meeting of the True Highlanders, held at Inverlochy, in Inverness-shire, on Tuesday 9th June 1818, published in The Inverness Journal, and Northern Advertiser (Inverness, Inverness-shire, Scotland) of Friday 19th June 1818 [page 3, column 1]—founded in 1815 by Alastair Ranaldson Macdonell (1773-1828), chief of clan Macdonell of Glengarry, the Society of True Highlanders promoted Highland identity and traditions:
The following Gentlemen,—Mr. Sutcliffe of Huddersfield, Yorkshire; Mr. Roberts of do. do.; Mr. Hooman of Kidderminster, Worcestershire; [&c.], presented themselves in front of the joyous circle, with bottles of whisky in each hand, and insisted upon permission to drink and hand round to the Health of Mac-Mhic-Alastair, and all True Highlanders, in a bumper of mountain dew.
4-: From The Morning Chronicle (London, England) of Friday 16th October 1818 [page 2, column 4]:
Sporting Extraordinary.—A grand hunt in the ancient style, was lately given to the True Highlanders, by the Founder of the Society, Glengary [sic]. It commenced at Invergarry on the 15th September, and ended on the morning of the 18th, when the Highlanders assembled on the brow of a hill to breakfast, consisting of a solid collation of all kinds of the finest moor game, washed down with choice mountain dew, (whisky).
5-: From The Inverness Courier, and General Advertiser (Inverness, Inverness-shire, Scotland) of Thursday 9th September 1819 [page 3, column 3]:
At the Marymass Market held here last week, little business was done, as the country people were generally employed at their harvest.—Two Irishmen who had been sipping pretty freely of the “mountain dew,” were taken into custody for riotous proceedings at the market. The Magistrates on examining the case on Monday, fined them a Guinea each, which was ordered to be given to the Treasurer of the Soup Kitchen, except seven shillings which were deduced for necessary expences.
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