Is it ensure, insure, or assure?
1721, "about to die," from Middle French moribund (16c.), from Latin moribundus "dying, at the point of death," from mori "to die" (see mortal (adj.)). Figurative sense of "near an end" is from 1837. Related: Moribundity.
moribund mor·i·bund (môr'ə-bŭnd')
At the point of death; dying.