Is it ensure, insure, or assure?
c.1600, "pertaining to mania; insane," from French maniaque (14c.), from Late Latin maniacus, from Greek maniakos, from mania (see mania). Borrowed at first in French form; Latinized in English from 1727. The noun is attested from 1763, from the adjective.
maniac ma·ni·ac (mā'nē-āk')
An insane person.