You mean as in
"One night she came to my bed side
when I lay fast asleep:
She laid her head upon my bed
and she began to weep.
She sighed, she cried, she damn’ near died,
she said: What shall I do?
So I hauled her into bed and I covered up her head,
just to keep her from the foggy, foggy dew." - ?
Partly it's pleasant-sounding nonsense, partly it's about the dangers to health of a damp climate, with fog and dew falling at night.
The "I" in the song is fudging the issue - of course he didn't just haul her into bed to save her from the night damps. That's why it sounds a bit weird and mysterious. But there is no secret meaning of "foggy, foggy dew".
Regards,
Sue.