The time of the month when a girl has her period. She becomes bloated has stomach aches, head aches, becomes moody because her hormones go fucking crazy (remember being a teenager?), and to top it all off, bleeds uncontrollably out her vagina. This can begin as early as age eight (Nature is an unforgiving bitch) or as late as the age sixteen (Nature taugths those who got their curse in third grade) Sometimes earlier or later.
Most men often do not understand this because they have never expirenced it and often say that she is just being a bitchy cunt because she can be.
(btw most guys that I met that even have a small headcold are really bitchy and lash out at every one so I think that men would be way worse then women at having PMS, but maybe thats just the guys I know)