Is it ensure, insure, or assure?
1610s, from Latin hystericus "of the womb," from Greek hysterikos "of the womb, suffering in the womb," from hystera "womb" (see uterus). Originally defined as a neurotic condition peculiar to women and thought to be caused by a dysfunction of the uterus. Meaning "very funny" (by 1939) is from the notion of uncontrollable fits of laughter. Related: Hysterically.