"Moon-dew" was somewhat of a "poison" used by the witches from Thessaly in their curses. It was comprised of a girl's first menstrual blood taken during the eclipse of the Moon.
I'm not sure exactly why menstrual blood has always seemed like such a "baleful" thing. The menstruating woman has been universally feared all through the ages. Originally this was because the onset of bleeding when there was no wound (See: Never trust something that bleeds for a week and doesn't die.) was taken as a frightening thing, and therefore thought to possess great magic.
This so called magic had to be kept out of harm's reach, and particularly away from men. A young woman at puberty became a "daughter of the night", as it was thought that during this time "no man could look upon her face and live".
In Inga Muscio's book Cunt, she says:
"Pliny said a menstruous woman's touch could blast the fruits of the field, sour wine, cloud mirrors, rust iron, and blunt the edges of knives. If a menstruous woman so much as laid a finger on a beehive, the bees would fly away and never return."
Pliny devotes a whole chapter of his Natural History to the subject and gives a long list of the powers for good and bad that a menstruating woman possesses. There is also an old myth stating that a man must never go near a menstruating woman during an eclipse, for he shall surely die. It seems ridiculous that such a natural thing should be so scary, and turned into "moon-dew", a cursing agent, but oh well, all the more power to the women.
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