I'm aware of the meaning of "scantily clad", the internet gives some good clues on that (Side question: Does it have erotic implications in itself?). However, what do the actual words mean ("clad" from "clothed"?), from which period is it, and why has it become a fixed expression?
"Scantily" is the adverb form of "scanty"
"Clad" does indeed mean "clothed"
So someone "scantily clad" is "insufficient clothed". The phrase describes (typically but not always) young women wearing lingerie.
That is, typically young, typically women, typically lingerie.
I don't know the first citation of "scantily clad" but it has become a useful cliche in common use today.
It is generally risque, typically (but not always) erotic. It is often used to describe "glamour photography"
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):
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):
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):
From "The Waterman of Besons: A Moral Tale," in The Universal Magazine of Knowledge and Pleasure (January 1793):
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.