Noun used to describe when you and your friend(s) are on your
period and thinking the exact same
thing, sometimes several times in succession.
It can also be used when not on
one's menstruation, but that was not what the phrase was originally meant for.
Julia: OMG,
Nancy, you know what we should do?
Nancy: Steal Bob's chocolate bar?!
Julia: Yes! We're, like, totally on the same vagina wavelength.