Randolph's "Ozark Folksongs" has a perfect transitional form, from "bugaboo" to "foggy, foggy dew," collected in Missouri in 1938:
"I wooed her in the summer time And part of the winter too. Tumble into bed, pretty maid, I said, And I'll keep you from the Boogy Boogy Boo."
As I mentioned on another thread, "foggy dew" was defined literally in 1616, without regard to the song (which didn't yet exist), as "rime" - a winter phenomenon.
Just what prompted the change is uncertain. But, in any case, the idea that the "foggy, foggy dew" cloaks some deep symbolic meaning is based on nothing. None of the collectors, and apparently none of the singers, ever suggested such a thing.
Since the original "Bogul maru" is quite literally a (fake) ghost, and since "foggy dew" is most easily understood as some kind of nocturnal meteorological dampness, there seems to be no room for arcane "symbolism."
Any presumed sexual meaning is unnecessary anyhow, since the maid's sexual intent is perfectly obvious - with the "dew" (or the "bugaboo" in other cases) being a mere excuse. (Her forwardness is the "fault" that the singer will never accuse her of.)
I believe Robert Graves was the first person to suggest that it did, and his argument that the maid was really fleeing the Black Death of the 14th century, is unconvincing, not to mention ludicrous.
The only "evidence" that "foggy dew" symbolizes something mysterious is that some people want it to. And for them, it can.