Is it ensure, insure, or assure?
mid-14c., from Old French eschec mat (Modern French échec et mat), which (with Spanish jaque y mate, Italian scacco-matto) is from Arabic shah mat "the king died" (see check (n.1)), which according to Barnhart is a misinterpretation of Persian mat "be astonished" as mata "to die," mat "he is dead." Hence Persian shah mat, if it is the ultimate source of the word, would be literally "the king is left helpless, the king is stumped."
late 14c.; see checkmate (n.). Related: Checkmated; checkmating.