The earliest matches for "scantily lad" that a Goggle Books search finds are from the late 1700s, and the first of these refers not to people's garments but to plant life in a barren landscape. From Johann Forster, ‎Observations Made During a Voyage Round the World (1778):

But as we advance to the South, and cross an immense ocean, in the midst of which we see some lonely birds, skimming the waves, and collecting an uncertain sustenance; we arrive at the South-end of America, and view a barren coast, inhabited by the last and most miserable of men, and but scantily clad with low and crooked shrubs: we find a number of vultures, eagles, and hawks, always hovering about, upon the watch for prey: and lastly we observe that the greatest part of the other birds live gregarious in a few spots; whilst the rocks are occupied by a race of seals, which in comparison with the rest of animals seem monstrous and misshapen.

Several instances from the early nineteenth century use the phrase in the same way, but the other three eighteenth-century matches involve people. From Robert Heron (translator), "The Idiot ; or Story of Xailoun," in Arabian Tales: Or, a Continuation of the Arabian Nights (1792):

He passed by a baker's door just as the bread was taken out of the oven, and was set out to cool in the tent. Its colour, form and smell moved his appetite. It was now winter ; and the day being cold, the heat from the oven was another inducement to him ; for he was scantily clad[.]

Coincidentally, the next instance of the phrase is also from Robert Heron, though I don't know whether this is the same person responsible for the translation of Arabian Tales. From Robert Heron, Observations Made in a Journey Through the Western Counties of Scotland in the Autumn of 1792 (1793):

Here and there [near Inverary] are farm-houses, not exhibiting indeed a very snug or comfortable aspect. Two or threes handsomer houses are seated in picturesque situations among the woods. The peasants seemed to be scantily clad ; and spoke only Gaelic.

From "The Waterman of Besons: A Moral Tale," in The Universal Magazine of Knowledge and Pleasure (January 1793):

Aleppo is a commercial city in the Levant ; and there I hoped soon to find some means of getting a passage to Europe. I was not deceived. But what I found there, that I did not expect, was my wife. The poor girl was a slave, and, with a crowd of others, was exposed to sale in the market of Aleppo, rather scantily clad, and with a veil over her eyes.

In any case, the first four instances of "scantily clad" in the Google Books database refer to a desert coastline, a poor man seeking employment, peasants in western Scotland, and a modest slave girl. Evidently, at that period, the expression was not especially closely associated with salacious ideas of near nudity.

The earliest matches for "scantily clad" that a Goggle Books search finds are from the late 1700s, and the first of these refers not to people's garments but to plant life in a barren landscape. From Johann Forster, ‎Observations Made During a Voyage Round the World (1778):

But as we advance to the South, and cross an immense ocean, in the midst of which we see some lonely birds, skimming the waves, and collecting an uncertain sustenance; we arrive at the South-end of America, and view a barren coast, inhabited by the last and most miserable of men, and but scantily clad with low and crooked shrubs: we find a number of vultures, eagles, and hawks, always hovering about, upon the watch for prey: and lastly we observe that the greatest part of the other birds live gregarious in a few spots; whilst the rocks are occupied by a race of seals, which in comparison with the rest of animals seem monstrous and misshapen.

Several instances from the early nineteenth century use the phrase in the same way, but the other three eighteenth-century matches involve people. From Robert Heron (translator), "The Idiot ; or Story of Xailoun," in Arabian Tales: Or, a Continuation of the Arabian Nights (1792):

He passed by a baker's door just as the bread was taken out of the oven, and was set out to cool in the tent. Its colour, form and smell moved his appetite. It was now winter ; and the day being cold, the heat from the oven was another inducement to him ; for he was scantily clad[.]

Coincidentally, the next instance of the phrase is also from Robert Heron, though I don't know whether this is the same person responsible for the translation of Arabian Tales. From Robert Heron, Observations Made in a Journey Through the Western Counties of Scotland in the Autumn of 1792 (1793):

Here and there [near Inverary] are farm-houses, not exhibiting indeed a very snug or comfortable aspect. Two or threes handsomer houses are seated in picturesque situations among the woods. The peasants seemed to be scantily clad ; and spoke only Gaelic.

From "The Waterman of Besons: A Moral Tale," in The Universal Magazine of Knowledge and Pleasure (January 1793):

Aleppo is a commercial city in the Levant ; and there I hoped soon to find some means of getting a passage to Europe. I was not deceived. But what I found there, that I did not expect, was my wife. The poor girl was a slave, and, with a crowd of others, was exposed to sale in the market of Aleppo, rather scantily clad, and with a veil over her eyes.

In any case, the first four instances of "scantily clad" in the Google Books database refer to a desert coastline, a poor man seeking employment, peasants in western Scotland, and a modest slave girl. Evidently, at that period, the expression was not especially closely associated with salacious ideas of near nudity.