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.