Pre-Menstrual Syndrome. An array of symptoms that comes from a hormonal upset as a woman's body prepares to either become pregnant or expel an unfertilized ovary. Can involve cramping, water retention, mood swings, spontaneous stimulation, etc.
As far as english speaking people are concerned, this is spoken by snooty types as to sound more sophisticated than they are, used humorously as such by people who *do* know what it means, and spoken by French when they need to say "I don't know what" in French.
Generally speaking, can be used if you're feeling particularly suave to give an explanation of something of a certain unknown or indescribable quality.