Hysterical means "marked by uncontrollable, extreme emotion." If your favorite sports team wins a championship, you might get hysterical and started weeping and screaming all at once.
Hysterical comes from the medical Latin word hystericus, which described a female neurotic condition, thought to be caused by a dysfunction of the uterus. Hysterical is still used today by psychiatrists, though in a broader way, to describe someone suffering from a psychological stress condition. Hysterical can also mean "extremely funny," even more so than hilarious.