Is it ensure, insure, or assure?
Old English sengan "to burn lightly, burn the edges" (of hair, wings, etc.), from Proto-Germanic *sangjanan (cf. Old Frisian of-sendza, Middle Dutch singhen, Dutch zengen, Old High German sengan, German sengen "to singe"). The root is said to be related to that of sing (v.), on the idea of some sort of sound produced by singeing (e.g. Century Dictionary), but Klein's sources reject this. Related: Singed; singeing. Singed cat "person whose appearance does not do him justice, person who is better than he looks" is from 1827.