Is it ensure, insure, or assure?
"action of restraining; means of restraint," early 15c., from Old French restreinte, noun use of fem. past participle of restraindre (see restrain). Sense of "reserve" is from c.1600.
restraint re·straint (rĭ-strānt')n.
An instrument or a means of restraint to prevent the infliction of harm to self or others, such as a straightjacket.
Control or repression of feelings; constraint.