The woman's "time of the month" when her uterus sheds its lining because her monthly egg isn't fertilized and dies.
The worst time to be around a woman, because she becomes an irrational psycho bitch which froths at the mouth with rage and seeks to destroy anything which stands in her way.
Since males don't have periods, they act like insensitive jackasses when the women around them are suffering from PMS. Those stuck-up dickless monkeys can't get any ass, so they call their girlfriends bitches and blow everything out of proportion. Sure, sometimes PMS can cause a girl to get a little bitchy, but those douchebags act like a PMSing woman is a nuclear bomb that's been unleashed on the city. All they think about is themselves, they don't even give a crap when their girl is crying out in pain because the girl is just "being a dramatic bitch".
A guy will accuse a girl of PMSing even if she gets mad at him for something that's his own fault, such as being a complete dickhole.
Bob: You bitch!! Why are you acting like a psycho?! You must be PMSing!!!
Sally: WTF are you talking about, you moron?
Bob: RUN AWAY SHE'S FOAMING AT THE MOUTH!!!!!! OH MY GOD SHE'S GONNA RIP MY LEGS OFF!!!!