What It Means Now:
Apparently, after seven years of marriage, the average man gets an itch in his junk that makes him want to go out, find a loose woman, and acquire himself a less metaphorical itch in his junk. This overwhelming urge to cheat after seven years of wedded bliss is referred to as the "seven-year itch" and was made popular by George Axelrod's 1952 comedic play of the same name. Said play was eventually adapted into the 1955 film starring Marilyn Monroe and panties, or at least that's the only part we remember.
Forgive us. We've been boringly married for over half a decade.
But It Used to Mean ...
Imagine that underneath her billowing white skirt is this:
Actually, that's a hand. The crotch is thrice as scary.
That, you suddenly lunchless ladies and gentlemen, is the original seven-year itch. The red, inflamed rash is an extremely contagious infection called scabies, and it's caused by evil little mites that burrow deep into the skin and cause an allergic reaction. Before modern medicine, scabies was known to plague those infected with it for damn near a decade -- and treatments never really prevented reinfection. So they called it "the seven-year itch" because "the infinite itch" provoked a few too many suicides upon diagnosis.
That means it wasn't an itch that happened every seven years; it was an itch that lasted seven years.
Because when you first see yourself with it, you break your mirror.