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"