Is it ensure, insure, or assure?
late 14c., from Old French dissimulation (12c.), from Latin dissimulationem (nominative dissimulatio) "a disguising, concealment," noun of action from past participle stem of dissimulare "make unlike, conceal, disguise," from dis- "completely" + simulare "pretend, assume, simulate" (see simulation).
dissimulation dis·sim·u·la·tion (dĭ-sĭm'yə-lā'shən)
Concealment of the truth about a situation, especially about a state of health, as by a malingerer.